Wavepad file has abnormal tune

[pourbaba]

I recently mixed different mp3 songs together using wavepad, and when i opened the file again the other day there was something wrong. The music had this kind of low tone to it, and wasnt sounding rite. All the files mixed are still mixed the same way, its just that the file sounds different and has a weird "tune". If anybody can help me it would be really appreciated. thanks