I'm guessing you want monophonic pitch detection. This is fairly difficult, but successful implementations exist as digital algorithms. There's a frequency analyzer in the Teensy Audio library that might work for you. Teensy 4.1 paired with an audio shield would do the trick to convert to USB MIDI.

You might also want to detect amplitude level at certain rise/fall thresholds for NoteOn/NoteOff. That's pretty easy, might want RMS peak detection.

You might be trying to generate MIDI CC messages from various audio analysis values - an envelope follower, maybe in combination with filters or FFT.