Me too. This isn't the first time this feature has glitched out. Even though I know its AI, I miss the coach.
I just wanted to add that I have sounds generally, like the pieces moving, so the problem seems limited just to the coach during review. This happened just after I had a message flash that review was unavailable. In fact, review was available, but the voice disappeared.
The AI voice has been working fine since yesterday. Today, it didn't work the whole day. I've changed the coach a few times because I thought it could be a small bug, but it continues not to work. It's beneficial to study a new language as well :/