Pass QEMU this flag:
-soundhw ac97
Or you could use (for a Ensoniq soundcard):
-soundhw es1370
However on some machines I get this error:
pulseaudio: pa_simple_new for capture failed
pulseaudio: Reason: Connection terminated
pulseaudio: pa_simple_new for capture failed
pulseaudio: Reason: Connection terminated
audio: Failed to create voice `ac9........