allow channel receive gain to be adjusted while recording messages/greetings...

allow channel receive gain to be adjusted while recording messages/greetings in voicemail (workaround for issue #2023 from the ancient past)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@6595 65c4cc65-6c06-0410-ace0-fbb531ad65f3