The RaceChrono's overlaid video export supports only AAC audio, but the video player supports also PCM audio and possibly others too. This is probably the reason why the sound is lost when exported.
You could convert the audio track to AAC before uploading the files to your phone, with ffmpeg command line tool that is available on PC/Mac/Linux:
Turns out it was a RaceChrono bug that appeared with video files recorded on Samsung S21. This will be fixed in v7.2.8. Please join the beta group to get the fix early.
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You could convert the audio track to AAC before uploading the files to your phone, with ffmpeg command line tool that is available on PC/Mac/Linux:
ffmpeg -i input.mp4 -acodec aac -vcodec copy output.mp4
Admittedly this requires some IT skills.