This is the basic idea;
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Use a standard D2A algorithm (check out the winmodem source in the linux kernel for instance) to record a sequence of timestamps onto a cassette tape, Compact Disc or even Vinyl record.
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Connect the output of your choosen playback device to audio input of a computer (Raspberry PI).
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Insert your media (casette) into your playback device and start playing.
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Continously decode the stream of timestamps, and use the timestamps to set the current position of audio playback on the Raspberry PI (probably playing MP3s)
Now you can use Fast Forward / Rewind to seek in your MP3s using your cassette player.
You probably also want to include Track ID and Tape ID in the code, so you can "map" specific casettes to specific collections (directories ?) of music. Now you can skip tracks (if using a CD player) and switch media to switch MP3 "collection".