New speakers cutting out/scratching?

I just got new yamaha 5.1 surround sound speakers for my birthday, and when I play rockband, I hear it sort of scratch every time a certain beat is played, like the b is too much for the little speakers. I was also watching Planet Earth and it scratched when the alligator attacked the wildebeast, but a lot of times it sounds just fine, is there a way to make it so no b comes out of small speakers and just the subwoofer? or is there something wrong with my new speakers? I have an old receiver, but it's only a couple years old and in good condition.