You can e-mail the recording to another PC and see if you can play it on that PC. If so, then it is a sound setting problem on your PC for playback. See forum post: When ONYX recordings have no SOUND, change audio device
If not the sound, play a sample music file with your Media Player to ensure your sound on the PC you e-mailed the recording to is working correct. If the sound is working with the sample file, then your ONYX did not record the file correct. See: Windows Sound settings for ONYX and USBClient
Go to your Sounds directory in Explorer -
this directory is located in the path specified as your "Recording Path" on the SETTINGS tab.
E.g. C:\Program Files\Datatex\Onyx\Data\Sounds\
Try to play one of these files. If you cannot open it, then you need to install Windows Media Player.