sip_audio_session: fix -S --disable-sound option

darcs-hash:20090327101612-b2873-2ffcc521226a66d3f82f5b68b87ea85fe0796f33
sip_session: add -S --disable-sound option

darcs-hash:20090327101807-b2873-ff135ab76e8154e74d7806548f5915ffd4de1b7e