Removed improper use of audio.silent setting from sip_audio_session

Ignore-this: 5c031d99bf92f65ab8e62093e9a48406

darcs-hash:20090825122128-1e3d8-b1b5046f982c665a6037e9f3ef65eb2d8a407b28
Renamed default audio device to system_default

Ignore-this: 69855b7594e72925ee4e800bdf30d5b

darcs-hash:20090825122145-1e3d8-0d91755025149e363e5c9f6f1e7790f8e7e39243