I realized that when I have this problem, on the iPhone the sound comes in mono. It only comes out of the handset. It doesn't come from the speaker, it's like I'm making a phone call. reboot does not work but I will try the method mentioned by @Robustez. making a phone call while listening to music may be the solution.

by the way, when you make a phone call while listening to music, sometimes the music starts playing behind the phone call. has anyone experienced this problem? (ios 17 had the same problem)