This doesn't seem to have an option in the Audio Settings image included, are you sure it's there?
Topic on Talk:Half-Life: Source
In most games its not actually an option. It just means that when you Alt+Tab out the audio is muted.
At the time, the audio settings section claimed there was a button to change it, much like there were in other Source games. That was what I was asking about.
Ahhh yeah, I mixed up Source and HL2 and just copied the notes from 2. Edited, thanks a lot.
Ooh, could have sworn the option was there. But yeah, it does that automatically.