WINXP sound problem

I can hear sounds from my WINXP computer if I plug in hedphones in the front jack or a really cheap non-powered pair of speakers into the back jack…. but when I use my powered speakers… 2 different sets were tried.. NO sounds ! Yes, I’ve tested the speakers. I must have some setting wrong but can’t figure out when I get unaplified sound but better speakers can’t seem to produce anything!
Any clues – suggestions will be greatly appreciated.
fastfinger at yippyiopc@yahoo.com