I've just done a little experimenting with Virtualbox to see if I could record sound into it with my microphone. ALSA (which I tested second) proved to be the most successful. Pulse Audio...

Well, I'm using Audacity Sound Editor for Windows within the guest machine. When you record something with it while Virtualbox is using the PulseAudio device, the sound records seemingly half (or other ever other) sampled audio. When you play it back, it's twice as fast and sounds choppy, but the pitch is kept the same. When you repeat the experiment with ALSA, no problems.

Not sure what to make out of this, but I guess I'll have to use ALSA for the time being.