As mentioned by AIQ, crank [something] means to make the something operate or function, especially by crank. The relevant device here is the stereo.
When it comes to music, we can say crank it up! to mean turn it up, turn up the volume:
To cause to intensify, as in volume or force: cranks up the sound on the stereo.
If you crank an engine or machine, you make it move or function, especially by turning a handle.
While this definition does not fit in perfectly, it can be used informally to mean to turn something on and then increase the intensity (e.g., crank up the radio).
Edit: The meaning of the phrasal verb "crank something up" is "to increase or ...
In addition to the possibility that it is a shortened version of "crank up" (i.e. increase the volume) suggested by the other answers, there's also the fact that old-fashioned record players were literally operated by crank. So perhaps the speaker is thinking of playing old Sabbath on something like the image below (credit: Fredrik Tersmeden, from wikimedia ...