We were having the same problem just now. My husband did a software update and that seems to have fixed the problem. What we will find out in the next couple of days is whether we have the same problem of the issue returning that Ronr399 has.
All the fixes you guys have listed I've tried and the reset sometimes works but it always returns within a few hours. This seems to be a growing problem, as there are now several threads of similar problems across several Roku models, and with virtually no help from customer service. My $$ is on a glitch in recent update they're working to fix.
I also have been experiencing the garbled sound issue on my 2-year old Ultra, did the update at the start of December, and it happened again just last night. The problem is temporarily resolved with a reset but comes back anywhere from a few hours to several days later, seemingly unpredictable. The problem started around August/September. I purchased a newer Ultra (so now we have two) that's been up and running for over a week with no issues. The older Ultra had been moved to another room and connected to an older HDTV after I purchased the new Ultra.