A virtual sound card can be attached to the host physical machine by using the sound element.

  ...
  <devices>
    <sound model='es1370'/>
  </devices>
  ...

The sound element has one mandatory attribute, model, which specifies what real sound device is emulated. Valid values are specific to the underlying hypervisor, though typical choices are 'es1370', 'sb16', 'ac97', and 'ich6'. In addition, a sound element with ich6 model can have optional sub-elements codec to attach various audio codecs to the audio device. If not specified, a default codec will be attached to allow playback and recording. Valid values are 'duplex' (advertises a line-in and a line-out) and 'micro' (advertises a speaker and a microphone).

  ...
  <devices>
    <sound model='ich6'>
      <codec type='micro'/>
    <sound/>
  </devices>
  ...

Each sound element has an optional sub-element <address> which can tie the device to a particular PCI slot, documented above.