Do you want to play along with a piece, but can't quite keep up with the tempo? Or are you trying to learn a tune by ear and can't quite hear what's going on? Audacity could be the answer!
Audacity is a free program you can download that lets you slow tracks down, transpose sound recordings and edit in all sorts of ways.
After downloading the program, click on "File" -> "Open". Find the track you want to slow down.
Make sure the Selection Tool is ... selected. (Looks like the "I" right beside the red record button.)
Click and drag to select the part of the track you want to slow down.
You can also do "Edit" ->"Select" -> "Select All" to select the entire track (Command⌘ + A shortcut on a Mac).
Click on "Effect" in the menu bar, then choose "Change Tempo". (If you choose "Change Speed", it will also change the pitch!)
Decide how much slower you want to go by sliding the button to the left. Click "OK".
Click on the green arrow to play your selected clip one time, or shift-click on the same button to play in an infinite loop.
And there you go!