A stray IR signal mutes the volume on the receiver and your Uverse remote is programmed to send all volume commands to the TV so you can't unmute it.
That troubleshoot app under the help menu can unmute the receiver. Order an extra remote and just leave it with factory settings to have it handy to send volume command to any receiver that gets its volume messed up. Or simply consult the manual for the remote on how to use the 955 code sequence to temporarily reassign the volume commands. »www.att.com/esupport/uverse-user ··· -guides/