I have this same problem with my Roku Ultra 4670. The sound will just stop working in apps (Disney+, Prime, Netflix). Only a restart of the Roku makes sound return. Please let me know how to solve this.
Same problem in October. Started 2-3 days ago. Built in roku in TCL TV. I have to rotate the audio settings to get the audio to initialize, hoping for a software/ firmware update in the near future to resolve this.