iRacing does this as well, and it's such a benefit to have voice comm from other drivers, as well as your spotter/crew chief barking in your ear, separate from the engine sounds over the speaker. Otherwise, it's a real challenge to try and balance everything over the speakers so you can hear clearly (actually, it's damn near impossible).
In the ingame settings, you simply tell the game what device you want to use for main game sounds, and what device to use for voice comm and spotter.
I would love to see this feature in ARMA2