Project based on SimpleAudioPlayer was sending me nuts , for 2 days. I wanted to play several files, or repeat them. The file would play OK first time, but next time would go in "slow motion". Designer of library forgot to tell us that you must every time Audio.begin(xxx,xx) before you start playing, and Audio.end() after you finished. If you don't "begin" every time, next file plays wrong, if you "begin" every time and not "end", it plays OK twice, and then locks computer.
Try adding an issue at wherever you got the library (Official Arduino library? Try GitHub)