Do a system recovery first. If its an Aspire One running windows you can enter the recovery console using Alt+F10 on boot up [when the acer logo appears, hold Alt and tap F10]. Warning! this will delete all data and programs that you have installed since purchase!!
If this does not fix the problem then it is more likely to be a hardware problem.
Try some earphones! is it cracking still when using earphones? Yes? then its the soundcard most likely. No? then its the speakers probably!
If its still apparent after a system recovery, and running windows update to check for a more recent audio driver, then you need to arrange service with Acer.