Reference
Why does my video have no sound?
A practical checklist for missing audio caused by codecs, tracks, surround formats, players, and conversions.
Quick answer: Silent video is often caused by an unsupported audio codec, a surround format the device cannot decode, or the wrong audio track being selected.
Check the audio codec
A video may play visually while the audio fails. This often happens when the device supports the video codec but not the audio codec.
Check audio tracks
Some files contain multiple audio tracks. Your player may select a commentary, silent, foreign-language, or unsupported track by default.
Safer conversion target
For compatibility, use AAC stereo inside an MP4 file. Keep a separate original copy if you need surround sound or archival quality.