Greetings. I have an A6 with Sirius. Yesterday the volume function ceased to operate and the "MUTE" icon is stuck on the screen. No sound on any stations. Instructions say push the on/off briefly to disable the mute function but it doesn't work. And.....when the car is turned off the speaker "sound" continues (a sort of hiss) even though it is muted. Any MMI wisdom regarding this matter?

Thank you.