VOIP Volume changing all on its own HELP

I have a hub abd spoke architecture in place with 1 main hub and 4
remote offices connected with VPNs and all are CISCO PIX515e's. The
problem i have is with 3 of those remotes reporting the volume level
goes up and down and get so low nothing can be heard at all. This
happens both internally and externally. ANY IDEAS?
Telecom, Cabling and VoIP continues...
Tags: Telecom VoIP Networking Internet cat6