you can check audio system stuff with alsamixer and then hit F5 on your keyboard to expand all the options. Then look for the Capture level and see where it’s at. I think I’ve defaulted that to like 87 in the disk image. (escape to get out of alsamixer). You will need to run sudo alsactl store afterwards if you made changes to get them to stick.
I believe the inputs want a line level signal so maybe a mic pre-amp would be needed in-between?
I don’t remember exactly. BT version is 5.50 on the fates install. I don’ t have a BT MIDI device to test with
bluetoothctl --help
bluetoothctl ver 5.50
Here’s some instructions for v5.48. Not sure if this is enabled by default in 5.5 or not. https://mclarenlabs.com/blog/2019/01/15/korg-microkey-air-37-bluetooth-midi-keyboard-with-raspberry-pi/
Also finding some of my previous research here: https://community.blokas.io/t/bluetooth-midi/152/8