diff --git a/src/app/api/audiobook/chapter/route.ts b/src/app/api/audiobook/chapter/route.ts
index 0a75004..15d7450 100644
--- a/src/app/api/audiobook/chapter/route.ts
+++ b/src/app/api/audiobook/chapter/route.ts
@@ -152,6 +152,46 @@ async function runFFmpeg(args: string[], signal?: AbortSignal): Promise<void> {
   });
 }
 
+function chapterEncodeArgs(
+  inputPath: string,
+  outputPath: string,
+  format: TTSAudiobookFormat,
+  postSpeed: number,
+  titleTag: string,
+): string[] {
+  if (format === 'mp3') {
+    return [
+      '-y',
+      '-i',
+      inputPath,
+      ...(postSpeed !== 1 ? ['-filter:a', buildAtempoFilter(postSpeed)] : []),
+      '-c:a',
+      'libmp3lame',
+      '-b:a',
+      '64k',
+      '-metadata',
+      `title=${titleTag}`,
+      outputPath,
+    ];
+  }
+
+  return [
+    '-y',
+    '-i',
+    inputPath,
+    ...(postSpeed !== 1 ? ['-filter:a', buildAtempoFilter(postSpeed)] : []),
+    '-c:a',
+    'aac',
+    '-b:a',
+    '64k',
+    '-metadata',
+    `title=${titleTag}`,
+    '-f',
+    'mp4',
+    outputPath,
+  ];
+}
+
 function findChapterFileNameByIndex(fileNames: string[], index: number): { fileName: string; title: string; format: 'mp3' | 'm4b' } | null {
   const matches = fileNames
     .map((fileName) => {
@@ -276,40 +316,36 @@ export async function POST(request: NextRequest) {
 
     await writeFile(inputPath, Buffer.from(new Uint8Array(data.buffer)));
 
-    if (format === 'mp3') {
-      await runFFmpeg(
-        [
-          '-y',
-          '-i',
-          inputPath,
-          ...(postSpeed !== 1 ? ['-filter:a', buildAtempoFilter(postSpeed)] : []),
-          '-c:a',
-          'libmp3lame',
-          '-b:a',
-          '64k',
-          '-metadata',
-          `title=${titleTag}`,
-          chapterOutputTempPath,
-        ],
-        request.signal,
-      );
+    const canCopyMp3WithoutReencode = format === 'mp3' && postSpeed === 1;
+    if (canCopyMp3WithoutReencode) {
+      try {
+        await runFFmpeg(
+          [
+            '-y',
+            '-i',
+            inputPath,
+            '-c:a',
+            'copy',
+            '-map_metadata',
+            '-1',
+            '-id3v2_version',
+            '3',
+            '-metadata',
+            `title=${titleTag}`,
+            chapterOutputTempPath,
+          ],
+          request.signal,
+        );
+      } catch (copyError) {
+        console.warn('Chapter remux failed; falling back to mp3 re-encode:', copyError);
+        await runFFmpeg(
+          chapterEncodeArgs(inputPath, chapterOutputTempPath, format, postSpeed, titleTag),
+          request.signal,
+        );
+      }
     } else {
       await runFFmpeg(
-        [
-          '-y',
-          '-i',
-          inputPath,
-          ...(postSpeed !== 1 ? ['-filter:a', buildAtempoFilter(postSpeed)] : []),
-          '-c:a',
-          'aac',
-          '-b:a',
-          '64k',
-          '-metadata',
-          `title=${titleTag}`,
-          '-f',
-          'mp4',
-          chapterOutputTempPath,
-        ],
+        chapterEncodeArgs(inputPath, chapterOutputTempPath, format, postSpeed, titleTag),
         request.signal,
       );
     }
diff --git a/src/app/api/audiobook/route.ts b/src/app/api/audiobook/route.ts
index e6abc47..afeae62 100644
--- a/src/app/api/audiobook/route.ts
+++ b/src/app/api/audiobook/route.ts
@@ -138,6 +138,13 @@ async function runFFmpeg(args: string[], signal?: AbortSignal): Promise<void> {
   });
 }
 
+async function ensurePositiveDuration(filePath: string, signal?: AbortSignal): Promise<void> {
+  const probe = await ffprobeAudio(filePath, signal);
+  if (!probe.durationSec || probe.durationSec <= 0) {
+    throw new Error(`Invalid duration for output file: ${filePath}`);
+  }
+}
+
 export async function GET(request: NextRequest) {
   let workDir: string | null = null;
   try {
@@ -230,16 +237,17 @@ export async function GET(request: NextRequest) {
       const bytes = await getAudiobookObjectBuffer(bookId, existingBookUserId, chapter.fileName, testNamespace);
       await writeFile(localPath, bytes);
 
-      let duration = durationByIndex.get(chapter.index) ?? 0;
-      if (!duration || duration <= 0) {
-        try {
-          const probe = await ffprobeAudio(localPath, request.signal);
-          if (probe.durationSec && probe.durationSec > 0) {
-            duration = probe.durationSec;
-          }
-        } catch {
-          duration = 0;
+      let duration = 0;
+      try {
+        const probe = await ffprobeAudio(localPath, request.signal);
+        if (probe.durationSec && probe.durationSec > 0) {
+          duration = probe.durationSec;
         }
+      } catch {
+        duration = 0;
+      }
+      if (!duration || duration <= 0) {
+        duration = durationByIndex.get(chapter.index) ?? 0;
       }
 
       localChapters.push({
@@ -268,31 +276,67 @@ export async function GET(request: NextRequest) {
     );
 
     if (format === 'mp3') {
-      await runFFmpeg(['-f', 'concat', '-safe', '0', '-i', listPath, '-c:a', 'libmp3lame', '-b:a', '64k', outputPath], request.signal);
+      try {
+        await runFFmpeg(
+          ['-f', 'concat', '-safe', '0', '-i', listPath, '-map_metadata', '-1', '-c:a', 'copy', outputPath],
+          request.signal,
+        );
+      } catch (copyError) {
+        console.warn('MP3 concat copy failed; falling back to re-encode:', copyError);
+        await runFFmpeg(
+          ['-f', 'concat', '-safe', '0', '-i', listPath, '-c:a', 'libmp3lame', '-b:a', '64k', outputPath],
+          request.signal,
+        );
+      }
     } else {
-      await runFFmpeg(
-        [
-          '-f',
-          'concat',
-          '-safe',
-          '0',
-          '-i',
-          listPath,
-          '-i',
-          metadataPath,
-          '-map_metadata',
-          '1',
-          '-c:a',
-          'aac',
-          '-b:a',
-          '64k',
-          '-f',
-          'mp4',
-          outputPath,
-        ],
-        request.signal,
-      );
+      try {
+        await runFFmpeg(
+          [
+            '-f',
+            'concat',
+            '-safe',
+            '0',
+            '-i',
+            listPath,
+            '-i',
+            metadataPath,
+            '-map_metadata',
+            '1',
+            '-c:a',
+            'copy',
+            '-f',
+            'mp4',
+            outputPath,
+          ],
+          request.signal,
+        );
+      } catch (copyError) {
+        console.warn('M4B concat copy failed; falling back to re-encode:', copyError);
+        await runFFmpeg(
+          [
+            '-f',
+            'concat',
+            '-safe',
+            '0',
+            '-i',
+            listPath,
+            '-i',
+            metadataPath,
+            '-map_metadata',
+            '1',
+            '-c:a',
+            'aac',
+            '-b:a',
+            '64k',
+            '-f',
+            'mp4',
+            outputPath,
+          ],
+          request.signal,
+        );
+      }
     }
+    await ensurePositiveDuration(outputPath, request.signal);
 
     const outputBytes = await readFile(outputPath);
     await putAudiobookObject(bookId, existingBookUserId, completeName, outputBytes, chapterFileMimeType(format), testNamespace);
diff --git a/src/components/AudiobookExportModal.tsx b/src/components/AudiobookExportModal.tsx
index d3d9fb8..14be830 100644
--- a/src/components/AudiobookExportModal.tsx
+++ b/src/components/AudiobookExportModal.tsx
@@ -76,6 +76,26 @@ export function AudiobookExportModal({
   const hasExistingAudiobook = Boolean(bookId) || chapters.length > 0;
   const isLegacyAudiobookMissingSettings = hasExistingAudiobook && savedSettings === null;
 
+  useEffect(() => {
+    // For new audiobooks (no saved settings/chapters), keep generation defaults aligned
+    // with the current playback controls so users don't need a route remount.
+    if (!isOpen) return;
+    if (savedSettings) return;
+    if (hasExistingAudiobook) return;
+
+    setNativeSpeed(voiceSpeed);
+    setPostSpeed(audioPlayerSpeed);
+    setAudiobookVoice(configVoice || availableVoices[0] || '');
+  }, [
+    isOpen,
+    savedSettings,
+    hasExistingAudiobook,
+    voiceSpeed,
+    audioPlayerSpeed,
+    configVoice,
+    availableVoices,
+  ]);
+
   useEffect(() => {
     if (savedSettings) return;
     if (audiobookVoice) return;