Hi Scarlett,
It sounds like the volume control has become worn out, hence the crackling noise when you turn it up and down. If the speakers are old and / or cheap I'd go out and buy a new pair. If they were expensive then you could try squirting some switch cleaner inside the moving part of the volume control. It may give you a few more weeks crackle free but in the end the volume control will need to be replaced.
For speakers I can recommend these Altec Lansing 1120's very good bass and treble response for around �17.
http://www.maplin.co.uk/Module.aspx?ModuleNo=2 18656&criteria=altec%20lansing&doy=24m8