As Superboto says, there is an option to change the tempo of the songs, the weird thing is that your songs play at different tempos..
Could you tell us what MPx do you have?
In my case, I can found the "change tempo" function in the MPx player which has a little speaker on each side, and has 2 earphone outputs..
While the song is playing, I press the joystick and a menu appears, there is one option called "tempo rate"
hope it helps
