I know this may only help 3% of the people, but thought I would share in case they read this.
I don't use in-game chat a lot. Normally talk with my crew in Discord. So, I don't know when this started... maybe when Season 8 launched.
My mic in Discord sounds just fine - however, when I tried using in-game chat to talk with other crew this past week no one could understand me. My crew told me it sounds like I'm scrambled.
While troubleshooting some issues with OBS screen recorder on my computer I found that my microphone HZ was not set to the same HZ that OBS wanted. In Windows 11 System ] Sound ] Properties my Input Format was set to 44100 Hz (CD Quality). OBS wanted it to match their setting of 48000 Hz (DVD Quality).
When I switched windows to 48000 Hz my Sea of Thieves in-game chat started working correctly!!!
So - I don't see a place in SoT game settings where it has a Hz setting. Not sure how my mic got changed to a Hz that SoT didn't like.
Glad I got mine fixed and hope this helps someone out.
I saw stream with HitboTC where he was trying to talk to a guy and no one could understand the other guys mic - it was EXACTLY what my mic was sounding like - so, I know I'm not the only one out there with this issue.