I do experience this as well, which is quite annoying. Sounds like broken earpiece speaker from the top of the phone but once replay it will become normal again.

Don’t cover the top of the screen. It will assume that you are pressing the phone on your face. The sounds comes out from top speaker.

Have the phone been washed before? With soap or something? My previous phone during the start of COVID-19 I did wash my phone daily... got once I guess the soap went into the speaker and caused the sound muffled. Eventually I had to wash several times and then it sounded good back.

I also used some of the app that emits sound to vibrate the liquid from inside the speaker out (similar concept with Apple Watch when clearing water from the speakers).