iOS comes with audio player that can play on double speed
without making it sound as chipmunk. But no options. 2x or 1x.
On Android, there is at least one free app that can provide
speed up, speed down, and pitch control. Nice.
Maple Lite for Audiobooks - Android Apps on Google Play
"MP3 player for audiobooks, podcasts, and lectures with variable playback speed.
Supported formats : mp3, ogg, wav(pcm)
* Browse and play your audio by folders, albums, and artists
* Variable speed (slow/fast playback)
Change playback speed (0.25x - 4.00x) with the same pitch
* Pitch Control
Change pitch from -0.5 to +0.5 octave (-6.00 to +6.00 semitones)"