Yeah Steam sucks like that when it comes to voice comms. You'd think a service with so many 1337 CS kiddies hell bent on screaming over voice would have implemented a better chat system.
Discrete output for game sounds + selectable input for input + selectable output for voice. Its been possible for years in XP. The Xbox had it, so does the 360. Many games allow for it as do TS or Vent.