Is the problem the card of the speakers?

I notice that funny noises sometimes come out of my digital speakers,
even when the computer is off. But my biggest problem is that the
speaker volume often swells and dimishes for no reason. It is not in
the content and when I use my earphones, attached to the analog out of
my Sound Blaster, the sound is fine. So which is more likely the
culprit, the speakers or the sound card?