We have a 7961 that we're testing our phone proxy solution with. We've had some intermittent audio problems and had a hard timing narrowing down the issue. But, I think we figured it out. I'm posting this to see if this is intended behavior or not.
When call recording is enabled on a line on the phone as well as the buit-in bridge, we get no audio to certain extensions (i.e. voicemail). Certain calls work fine, but other calls have no audio. When doing a "show phone-proxy media-sessions", no SRTP session gets established between the phone and the ASA when the no audio issue exists. The no audio seems to be to certain endpoints and is reproducable. If we either disable the built-in bridge or disable call recording on the line in question, everything works. In addition, when the problem exists, the problem is specific to a line on the phone (which may indicate the issue is actually with the call recording setting being enabled).