As Jeremy said, it really depends a lot from the chip you use. What card do you have? Is it external card (as in a PCI - card), or an integrated chip such as AC97 aaudio used by ie. VIA and Intel boards?
Case 1. External card.
Exellent possibilities to get it working how ever you want. depends a bit from the chip. (I suggest SB Live.. Have Audigy 2 ZS myself..)
Case 2. Integrated chip.
Don't even try it. (Or if you do, you're gonna be asking for trouble..)
I believe you want it to work as following: Game (such as WoW) + VoIP - software (Ventrilo, TeamSpeak, etc)..?
Only way that you get that with an integrated chip could be possible useing a third-party commercial software such as OSS. (the new one, old was in kernel like Alsa nowdays is..)
Reason for this problem is, that in Linux the audio software works a bit differently then in windows, from my experience (WARNING, a GUESS on it's way..) with internal chips is that Windows uses software to get those work full-duplex mode. (correct if wrong..) PCI cards like ie. the one I have (audigy 2 ZS) has built-in hardware to use full-duplex.
(Full-duplex == many sound channels at once)
Linux drivers "lock" the audio software so, that if one software uses the chip, others can't send sounds in.
I used demo version of OSS to create 2 fake-audiocards that took Game sounds to one and VoIP to other, and those combined audio and sent it to the real card. But it took around 2 weeks to get it to work, and a whole lot of coffee and tabaccos to achieve. Then I bought SB Audigy ZS and it works all right away.
-Thautane-
P.S Jeremy, correct me if I had wrong/old information, Had to go through that sh*t so long time ago that luckily I've forgotten most of it..
P.P.S For you who have the problem; post here what cards you use. The model and the chip it has. Let's search for a solution when we have more info.
Cheers.