I find the sound problem really annoying in a mac that works quite well as a video streaming client. I found the audio board 820-2364-A of the 2009 imac in the bay . I’ll get back to this thread, if that does the trick.
Reg bluetooth I did both fresh install of Mojave and upgrade from High Sierra and re-ran post-install several times - all with experiment character. As soon as I have Mojave bluetooth is gone. I got around that with an ext iogear usb dongle.