Most computers have a line in socket. If you have Windows, just get a 3.5mm to 3.5mm cable and use the free "Audacity" application to record the audio. . It'll give you a wave file. Can then use foobar2000 (or whatever) to convert to mp3 if you want.
There may be easier ways, but that's how I converted some last century tape recordings to digital format.
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