An even easier fix is to just press the '*' (star key) on the Roku remote during playback of an active stream where the audio is messed up...This will bring up the 'Options' menu where you can then cycle the volume modes ('Leveling', 'Night', and 'Off')... going to the 'Off' mode seems to be when it resets the audio and fixes the problem.
Once the audio gets reset, you could then go right back to the 'Leveling' mode again (if you like)...
I wasn't sure that "garbled sound" was the right description for my problem until someone said it sound like there are demons in my TV. Then I knew I was in the right place. As everyone has suggested, any form of switching audio settings or power cycling fixes the problem for a random amount of time. But I don't think the problem is the ROKU. This happens on my new LG 86 inch TV with an ARC connected (HDMI) pioneer surround system. I don't own a ROKU. I feel the problem has something to do with how new devices decode surround channel steering logic. This did not happen with the same audio setup and my 5 year old TV.
I don't know because I'm not familiar with decoding. I bought my TV in 2016, Vizio Smart TV, and now have a ROKU device. I haven't had problems with the sound for a long time. After I posted this, it seemed as if I never had it. I think my TV sometimes updates, so maybe you're right. Maybe it's the TV, not the Roku.
Push your asterisk button. There you will be able to scroll up and down. You will find volume settings below caption settings and audio track. I moved from one setting to the next and keep mine on the night setting. I haven't had garbled sound for a long time. Hope it works for you.