I don't mean to imply that the other suggestions here are incorrect (otoh there's no guarantee that adding capacitors across the supply rail will fix this, it might but likely won't).

But given that the original poster had to ask the question (anyone who knows how to fix this wouldn't need to ask the question), and that instability in amp circuits can be tough to track down even when you know this stuff, and that computer speakers are typically $20 -$30 -- I really think the best thing to do is just toss them, if basic troubleshooting confirms that the speakers are indeed the problem.

my 2 cents...