Tell a colleague, kindly

People can't hear their own muffled mic or see the window behind them, but you can. This makes a friendly link that points them at the fix, so you don't have to spell it out.

We know it can feel awkward to bring up. Most people would still rather hear it, and chances are you're not the only one who's noticed. Once it's fixed, every call is easier for everyone on it, including them.