Track Studio helps you learning and transcribing your favourite tunes by changing the speed and pitch of the track in real-time. You also have access to a handy "skip back" button and AB looper, to get through the trickier bits.
Please note: since the app needs to process the full audios waveform, the song needs to be available on your device. Unfortunately, the most popular streaming services dont offer access to the whole audio file to third-party apps since all files are protected by Digital Rights Management technologies :(