Maple Player Classic audio player is easy to use player which has following advantages:
1) Browse and play your audio by folders, playlists, albums, artists, and genres
2) Tempo control (variable speed playback) Change tempo from 0.25x to 4.00x with the same pitch
3) Pitch control Change pitch from -0.5 to +0.5 octave (-6.00 to +6.00 semitones)
5) Auto playback history
7) Sleep timer
The best part is you can start playing all the mp3 files for a particular folder and speed up the Tempo for faster playing. This is very good for reading books.
You can download the app from https://play.google.com/store/apps/details?id=com.sqr5.android.player_jb