We've been experiencing this issue ever since we started using the Roku Ultra models. Hooked to 3 different TVs with 3 different sound systems.
We had 3 of the first Ultra versions. Every time you pause too long, and sometimes just randomly- Roku loses sound. Sometimes video will freeze too- but always lose the sound. The only solution is to reboot. I've tried all the updates and settings tricks found online to no avail.
So I gave up and bought 3 of the latest Ultra models last month. Frustratingly- 3 brand new units have the same issue right out of the box.
We currently have a Linksys Mesh system + Netgear router setup (so I can separate family and work networks). Before that we had just a router - first Asus, then a Netgear. So it's happening on different Network setups as well. I do pretty intense development work so buffering is not an issue on other devices on a regular basis.
It seems to be related to buffering in the Roku; when a pause is too long. Or perhaps it's getting small power surges or something else hardware related it just can't handle.
Someone in Roku QA needs to sit and watch a program (YouTube TV is the worst), then pause it for a good while. Half an hour or more. Don't reboot, ever until you have to because the sound stops working. Really put some effort into diagnosing this. Or Roku is going to lose customers over this annoying problem. I'm not buying another Roku until this is resolved.