Fix: re-encode video in mux_video_audio instead of stream-copy to handle any input codec

app.py

@@ -286,7 +286,10 @@ def mux_video_audio(silent_video: str, audio_path: str, output_path: str,
             ffmpeg.input(silent_video),
             ffmpeg.input(audio_path),
             output_path,
-            vcodec="copy", acodec="aac", strict="experimental",
+            vcodec="libx264", preset="fast", crf=18,
+            pix_fmt="yuv420p",
+            acodec="aac", audio_bitrate="128k",
+            movflags="+faststart",
         ).run(overwrite_output=True, quiet=True)