no, all of the above advice seems wrong to me.
damage of this type always carries with it specific behaviors, which you do not write about, and it is really difficult to overlook it. Therefore, I believe that there is a broken connection of the motherboard and/or a short circuit in the boost circuits for the audio chips (the iPhone has three to four chips related to audio processing, each of them can cause similar symptoms). If you're not into microelectronics, don't do it yourself. Find a good repair service and order the diagnostics. On the other hand, this hypothesis will be confirmed (because you don't provide anything to prove this fault) only by the start.log or em.log (which in the latter case does not have to be so certain).
If I'm right, in a normal working good repair shop it shouldn't cost much. The time needed to perform the diagnostics of this device is about 10 minutes, the repair time will be 30-40 minutes. Total, about one hour.
Call somwere, find out, decide... you will surely find such a place.