Dips in sound

well, whilst I've been listening to music recently every now and again it goes really quiet and i can't hear it properly then it pops back to normal volume...which you can image is really annoying...I can't seem to work out what's causing this, but does it sound like something due to i-tunes, pc settings or the speakers themselves :/

do you have Dell speakers? I have the same problem with mine, they're really junk. I can fix it usually with an adjustment to the volume knob on the speaker, it'll just pop back to normal.