PulseAudio is GNOME's low-level audio API. It is what sits between applications and the kernel's audio API. PulseAudio lets you re-route sound through the network, or to a Bluetooth headset. It can mix several sources of audio, or change the sample rate of an audio stream.

All the parts of GNOME that produce audio use PulseAudio in one way or another, either directly, or indirectly through higher-level sound manipulation APIs like GStreamer.