Improved volume control on ConferenceBridge

Ignore-this: b085477445270238f4dcfe2232bf8f5

darcs-hash:20090723144744-1e3d8-9904f9fb6620e9a4f8f24a7249cab51b29889e75
Added mute feature to sip_audio_session

Ignore-this: a80c137018aca46b030b261da9c61d5b

darcs-hash:20090723144750-1e3d8-ced548f50ea8610bfc3ab461be013df67812f5a4