Why do the internal speakers crackle and distort?
The internal speakers on my sister's 2009 macbook white (a1342) struggle to produce distinguishable sound - on low volume they just gurgle and crackle quietly and on high volume the sound is quiet, distorted and regularly interrupted by loud crackles. Even when the volume is off, things that trigger a sound if the volume is on (such as putting a file into the trash) make the speakers whir and gurgle. They also carry on whirring for a few seconds after the sound should have stopped.
I bought the laptop off ebay spares and repairs needing new battery and hard drive and general cleaning up so unfortunately I don't know what the last owner might have done to it although he says he never noticed it as he always used an external speaker.