Audio not working on Video

Hi All,

just wondering if anybody has come across this before:
I imported an MP4 onto an audio track, mixed and synched etc…, done the audio mixdown, replaced the audio in video file but when I play back the new MP4 in Windows Media Player there is no audio coming through?
Maybe it’s codecs?
How do I identify codecs issues if this is the problem?
Many thanks,
Mmm