This application will allow you to select a fragment from a song, change pitch and speed and play it in loop. You can then play along and learn how to play more difficult parts in a more easy way.
Changing the pitch and speed will require you to optionally buy a Pro add on. In addition you can also boost the right instruments using 4 band equalizer and select left or right channel to play separately. The Pro version will also allow you to save the fragments and thus quickly move from one part of a song to another.
You can try the pro features for free for 14 days.