Cracks and pop sounds with sound cutting in and out usually indicate a loose wire/bad connection to the speaker. Sometimes it can also be caused by electrical interference, but the huge difference in time interval you're reporting suggests this isn't the case. I really don't think software is the cause, but you can test it by plugging in headphones. If you get the pops and dropouts over the headphone as well, then it's a software or sound card problem.
Assuming the problem is a speaker wire, open up the laptop and check all the wires going to the speakers. Usually it's a bad solder joint right where they connect to the speaker, and a quick touch-up with a soldering iron will fix it. The more difficult cases are when the bad connection is due to a frayed wire - you need to find where it's frayed and either fix it or replace the wire. The most difficult cases are when the problem is in the connection with the motherboard. Frequently there's nothing you can do if that's the case, as the pins are usually too small to solder.