You've received a lot of advice so here's a bit of history and why it is great to want to "slow down music today" vs yesteryear. With the digitization of music, it is possible to slow the music down but keep the pitch the same....something that wasn't commonly available to the public. By the early 1960's radio stations had devices called "rate changers" to use with there analog reel to reel recorders. These were used to "fit" commercials into a certain length of time within parameters. A rotary head "sampled" out pieces from the recorded tape to accomplish this. For the rest of us in the 50's, 60's, 70's and into the 80's, the method to slow down music to learn was to either get a record player that would play a 33 1/3 rpm record at 16 2/3 (and the pitches would all be an octave lower) and you would hear it at 1/2
normal speed or to acquire a reel to reel tape recorder and then play the recording at 1/2
the speed it was recorded, again lowering the recorded material by one octave. Reel to reel recorders in those days commonly had speeds of 1 7/8 ips, 3 3/4 ips, 7 1/2 ips and 15 ips.
There were machines that recorded as slow as 15/16 ips and as fast as 30 ips. Then there
was a variety of head formats (in other words, if you had your recording at 7 1/2 ips and wanted to play it on a friends machine, it had to have the same head format to work properly). There were full track, half track, quarter track, half track stereo, quarter track stereo and other formats.
A very popular device for music students in the 70's and 80's was the Marantz cassette recorder. Portable and designed with musicians in mind, it offered a 1/2 speed playback along
with a variable speed playback...but still the pitch was changed.
Soooo... we are very fortunate to have today's options at our desposal.
When I think back to my youth when most households had just a radio and very few a tv,
today's world is more than I ever could have imagined.