usb audio clock question

[mattes]

GStreamer is typically using the audio clock to provide 
synchronization for Playback and Recording of a media AV stream.

I was just wondering if one would to use a USB audio dongle,
instead of the internal built-in audio, which clock is actually
to be used. The actual audio playback device is located on 
the other side of the 'usb' wire.
So there is no direct access to the clock of the USB audio chip.

In such setup, which clock is used by GStreamer?

Mat