I have the same issue I know it is al old phone but still it was working good went up to the mountains got discharged and then exactly the same problem as the OP ... tried most of what is suggested here tried to update the IOS but at the end it says Phone ineligible for this build .... What now???
Going to an Apple store has no worked for me and I went to a couple to sample... all the guys(geniouses?) want is sell you a new phone -) I have the same issue it was working good went up to the mountains battery got drained and then apple logo and black screen..tried n all of those tools and they do nothing to my phone even the DFU starts but then it says it cannot do it
Had the same issue took it all apart & put it back nothing, sprayed the contact cleaner ,pulled the speaker out tested it outside it was good, then i saw where the speaker sits if you push the cover you'll hear a click that is the notch that pushes the cover & makes the contact.
I swapped the screen on a 6S and had to get the Touch ID from the broken one to get it to work … I took a couple of quick shots but didn’t work didn’t feel like resetting my iPhone to go any deeper
I had nicely paired but not sound at all..disassembled it completely &put it back not sure if it was a bad connection but putting it back &all is well. Each side seem to have 2 spkrs but the longest speaker looking has no connection to the board so not sure if they r using it with some audio effect or something else