Having the same issues here w/ v11.5 - just updated from 11.0 last night. Roku Ultra 4k that's been in place for ~3 years now, hardwired to internet (400mb) and then HDMI to Anthem AVM-60. Haven't had any issues in the last 3 years with it.
Now getting intermittent quick (<~1 sec) audio drop-outs / hiccups ever 20-30 seconds - as well as the lip-sync falling out of sync. It's honestly unwatchable at this point.
Definitely something in the 11.5 upgrade... Honestly Roku needs a way to rollback to a previous firmware version as a short-term fix and their team here to bug-fix 11.5 to figure what's been screwed up from 11.0
Also worth noting that ARC volume control of my Anthem AVM using the Roku remote has been flawless and now is not working since the 11.5 upgrade last night.
I tried a factory reset of the device (which sadly had it auto-upgrade back to 11.5) and the problems remained the same. For now I've only tried Peacock channel - as I was too annoyed last night to try more... 😞