Keep in mind that these tests were conducted on debian lenny with a backported version of kvm and qemu. Some of the sound capabilities were improved after debian lenny was released so this may not work with the default package versions. This will also be the case for ubuntu intrepid. The newer version of kvm appears to be available in ubuntu jaunty.
There are two dimensions to configuring kvm to work on your system:
1. Emulated sound-card interface to the guest operating system
This is handled, for the most part, using the -soundhw option on the kvm command line. By changing this option, you can preset different emulated devices to your guest OS.
2. kvm access to your linux sound system
This is handled, for the most part, using environment variables which are set prior to invoking kvm.
There are several available, selected with the -soundhw option. The most popular seem to be ac97 and es1370.
see: kvm -soundhw '?'
see: kvm -audio-help
The audio driver in the host is configured with environment variable: QEMU_AUDIO_DRV
Supported values: alsa, pa, sdl, oss, wav (other values: esd, core, dsound may also work)
if not set, default is ? How to find out which default is in effect on your system?
Examples
With my system I was table
example with winxp:
/usr/bin/kvm -m 3584 -smp 1 -name xppro09c -localtime -boot c -drive file=/mnt/luks1/kvm/xppro09c/xppro09c.img,if=ide,index=0,boot=on -net nic,macaddr=54:52:00:7c:4d:25 -net tap -usb -usbdevice tablet -k en-us -soundhw ac97
https://bugs.launchpad.net/ubuntu/+source/kvm/+bug/304649
Discussion on kvm and pulseaudio issues, bugs and some enhancements coming in kvm package version 1:84. This is part of the reason I decided to backport a newer version of the packages into debian lenny.