I don’t have issues with the microphones and call quality, but I completely agree that the speakers can be pretty awful.
The top one is especially painful to listen to 
, took me only seconds of listening to a piano piece before I went to Accessibility Settings, switched the output to mono and shifted the balance to the bottom one.
Not perfect, but waaaay better.
I had to do them same on the FP4 of a family member, seems like a general flaw to me.