This is well documented, unfortunately.
You need to try all the usual methods, re-connecting everything, rebooting, resetting the whole shebang.
Most people go back to a non-ARC audio connection, which usually means digital optical from HDTV to receiver for the more reliable functionality. YMMV.
I have suddenly lost TV sound through my AV receiver/speakers. Have connected through HDMI2 (ARC). It worked fine first week but now its gone. Everything else works and I simply can't find any wrong settings. Could anyone help me?