Rear Speaker problem with unibody white macbook

Hi everyone!

I have a mid2010 unibody white macbook, and since 1 month ago, the rear speaker started sizzling. It onyl sizzles but no sound! I have ordered a new rear speaker and replaced it but nothing changed! I connected the 2 rear speakers to left speaker plug to confirm they work properly, and they both worked. What may be the problem? Is there anyone who experienced this before?

Thanks!