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 `ac97.pi'
pulseaudio: pa_simple_new for playback failed
pulseaudio: Reason: Connection terminated
pulseaudio: pa_simple_new for playback failed
pulseaudio: Reason: Connection terminated
audio: Failed to create voice `ac97.po'
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 `ac97.mc'
linux, qemu, kvm, enable, soundcard, guestlpass, soundhw, ac, ensoniq, es, pulseaudio, pa_simple_new, capture, terminated, audio, pi, playback, po, mc,