I am pretty positive it has to do with the bass, some songs the bass is too much , or if it is a burned cd , the bass might not be a good quality which makes the speakers sound ugly, especially if they are just stock, if you have aftermarket speakers which can handle alot of bass it would sound cleaner. It also depends on the type of music, like alternative or easy listening songs sound fine , even with the volume all the way up, but with some rap songs the bass gets to be too much for the speaker at times