Audio sharing

When somebody is playing audio using the "screen sharing/advanced" facility, we've noticed that if a participant speaks, sometimes the audio cuts out (or at least goes much quieter) until they stop, and then is restored after a few seconds; but other times it doesn't seem to do this.
Does anybody know what governs this behaviour? Is it under the host's control?
Obviously sometimes it is desirable, in order that people can be heard; but if you're playing quiet background music, for example, it's much more distracting to have it cutting in and out than it would be to let it play.