Chris Randall (Audio Damage, Sister Machine Gun) shared this video that demonstrates how to add voltage control to a variable speed cassette tape deck.
Inspired by the Onde Magnétique, Randall opened up a variable speed playback tape recorder and added an input jack that essentially do the job of the playback speed knob using control voltages. Once the control voltage input is added, you can control the playback speed & pitch of the tape recorder using a modular synth.
If you record a sustained note or chord onto the audio tape, you can then play it back with the modified tape deck like a voltage-controlled monophonic Mellotron.
One quirk of this hack is that the cassette recorder is not designed around standard modular control voltages, so you have to be able to dial in whatever input voltage is needed to get the playback speed you want. This is easy with Randall’s Audio Damage Sequencer 1 or traditional step sequencers that allow you to work with unquantized voltages.