Curious!
I'm running IL246 on my Vista 64 Ultimate system just fine. Teamspeak also works just fine.
However, if I'm on TS with my squaddies, then start IL2, I can still hear incoming audio from my mates, but they can't hear me. If I minimise (not close) IL2 everything is fine again. Once it's maximised, my mike goes dead again.

The audio is Realtek on-board in my EVGA 780 SLI m/b.
I did find a setting buried in the mixer which said "allow priority application to take control of the microphone" and switched that off, but the problem persists.
The voice comms in IL2 is switched off of course.

Any ideas m8s?

May not be related but I had this once, turns out that under some circumstances if you have the ingame mic level turned all the way down it can turn the windows mic level down, so make sure the ingame comms switch is off but the mic knob is all the way up. See if that helps.

I'll try that and report back. Thanks Shinjiro

Tried that, no joy.
Tried disabling on board Realtek sound and put my old Audigy 2 in the new computer. Got the CReative Labs Vista 64 drivers.
Problem persists (and sound quality is worse than the Realtek!)
Mike still mutes when I maximise IL2

Some research dug up the answer.......

Either
1) Switch off UAC
2) Teamspeak and ventrilo must be "Run as administrator"

In other words, it's not a bug, but a "feature"

I should have thought of that. I guess I'm so used to turning off UAC first thing that I forgot about it .