I've experienced this a few times with my UF770V set.
First time, had the mainboard replaced under a support call.
Second time, it was after I'd been messing with settings - found that ARC wouldn't work with HDMI Deep Colour enabled on HDMI1. Disabled it and ARC started working again.
Third time, seemed to coincide with the update to 04.25.56 firmware. After a *lot* of messing about with resetting to defaults, leaving unplugged, changing ARC/HDMI control settings on the TV and sound bar, I discovered that the following fixed it for me.
Turn HDMI control off at the TV and soundbar
Turn TV and sound bar off and unplug from power
Disconnect all HDMI leads apart from the one between the TV and soundbar
Plug TV and sound bar in, and turn on
Enable HDMI control on the sound bar
Enable ARC and HDMI control on the TV. Sound output to the soundbar show now work
Turn the TV off - soundbar should turn off with it
Turn the TV back on - soundbar should turn on automatically
Plug the rest of the HDMI leads back in and test again
Also, I found that I had been setting the TV to PCM output, which basically meant stereo output even when the source was 5.1. Leave as Auto rather than PCM.