There seem to be quite a few topics floating on the support forum regarding problems with the audio developing an "underwater" sound or getting very distorted. I've been experiencing this on my Roku Ultra quite a bit lately. The app that I mainly use is the Xfinity Streaming App and it is defintely present there. I don't use any other apps enough to say if the problem is specific to the app or not. However, as someone who has spent 20 years in tech fields, I do have some opinions here.
First off, I have tried all the obvious stuff. Swapping HDMI cables, Trying different HDMI ports on the TV, playing with audio settings, etc. etc. etc. The problem is CLEARLY tied to the Roku device as the TV has no audio issues what so ever when streaming directly from the TV apps, using a Blu Ray player, or using a cable set top box. Rebooting the Roku device does "solve" the problem, but the problem eventually returns so it's not really solving anything - it's just band-aiding it. Others have said they can tinker with the audio settings in the Roku menu to get the problem to go away. That hasn't been the case for me, but when audio settings are adjusted, the Audio Chip / Amp IC circuit within the Roku most likely quickly resets itself as it moves from one audio mode to another. That quick reset of the audio chip when toggling between audio modes is more or less very similar to a full blown reboot of the Roku device - at least from the standpoint of troubleshooting audio problems. None of these fixes are actual fixes. They are just workarounds and inconveniences for the customer. I think the clear problem is a subpar audio amp within the Roku device. It's a hardware problem. Roku won't admit to this as the problem, but given the nature of what happens when you toggle audio settings (quickly resetting the audio IC) or when you reboot the entire Roku device (also resetting the audio IC, a bit more thoroughly) - this is a hardware problem. Is the audio chip overheating? Is it not properly processing the audio format of streaming apps? Who knows - but none the less, it is a HARDWARE PROBLEM. Unfortunately, Roku won't admit to this as they'd potentially be on the hook for an actual fix at that point, but the conclusion to be drawn here is there isn't an actual "fix" to this. Just work-arounds. And I'd urge everyone to keep that in mine when it's time to replace your Roku devices. Does fire-stick have this same problem? If not, maybe that's the direction to go.