If you recorded multi-track audio in an mkv or mp4 file, the player will only play back one audio track at a time; you'll have to select which one is active in the player interface. This feature is for editing in software that supports multiple audio tracks; most players don't.
If you didn't intend to record multi-track audio, then it's possible your microphone is assigned to a different track than the one you recorded; check this in Edit > Advanced Audio Properties.