I have the runaway volume issue too. It happened for a few weeks, seemingly got resolved, then started again a few days ago. This issue, combined with the previous issue of the remote sporadically no longer controlling volume at all, gives me the impression of loose software quality control at Roku.
I have an LG OLED C1 with a Vizio soundbar connected via HMDI ARC. Everything was good for a long time then these glitches started plaguing our experience. I'm extremely frustrated. I have not had time to find a suitable alternative for my multiple Roku Ultras but my wife is done with them.
Coincidentally, my personal news feed includes investment tips that suggest its a good time to buy Roku stock. No thanks. I only invest in things I have confidence in.
Mine did the same thing, support even sent me a replacement but that one also started running away, even started changing 4 or 5 channels at a time. After this support was unresponsive. My solution was to purchase a 14.00 rca universal remote with streaming device compatibility. I lost the voice capabilities but it fixed the failure of a 100 dollar failing device, roku support is a joke, they rely mostly on there customers to troubleshoot and repair there devices and very rarely come up with a fix for anything.