[evanevery]

An Arduino or ESP32 with a CANBUS board/shield would allow you to grab the CANBUS info (RPM) and work with it. (The ESP32 has onboard Bluetooth!)

Add a Wav or sound generator board/shield to control in response to the RPM and you could pretty much play back anything you want. Feed it to the BMW amp via the Black and Black/Grey wire and it comes out the back of the car.

I'm not sure if just speeding up the playback of an audio sample in response to the engine RPM would be good enough. I would want to be able to play with various aspects of the sound sample (loudness, compression, bass, treble, reverb, etc) at the same time..