You need to take a look at your audio configuration.
Right click the small speaker icon usually located in the lower right hand screen corner.
Go into "Playback devices" and drill down through all of the various Windows, tabs, and properties.
Take a very careful look at all of the settings and see if any of the settings seem to be amiss.
Correct/change as necessary but only one at a time. Keep notes in case you need to undo some change.
My thought is that Steam is expecting audio from one source but your system is configured to use another audio source.
Check the game settings as well.