Portable cassette tape players aren't just the best way to listen to music on the go, they also can be turned into musical instruments in their own right! By adjusting playback speed of the tape motor, you can increase or decrease the pitch of the playback.
This guide shows a couple of great ways to hack your Walkman for speed control. First there's the Walkmellotron, which uses USB MIDI on a QT Py RP2040 with a DAC board for voltage control of the player's potentiometer wiper. Use a MIDI keyboard, controller, or sequencer to "play" tape tunes.
The second method replaces the tape player's on-board motor controller entirely, allowing total motor control, including a wide range of speeds in both forward and reverse! NOTE: custom tape loop cassette required.