Added volume argument to PersistentTones

Ignore-this: daaf55cc6e29dc271f44ddbf3ab86b3e

darcs-hash:20090825094949-1e3d8-38c869a42e2cf4cd5f3fda3583695b4b59bfb403
Changed volume of hold tone in sip_audio_session to 50%

Ignore-this: 447e2fa56e002d0ed306cbc6772f816c

darcs-hash:20090825095007-1e3d8-51339b24030eabcc27d48e4378955929c163c4f7