External speakers won't work when plugged in to...

Question

It's on a laptop: both small laptop speakers work, but when i plug in external speakers or headphones in the headphone jack then the sound only comes out of the Left one (or nothing at all). My dell guarantee has expired so they wouldn't help me out. What else can i do and at what cost? and whats the problem? sound card? it happened after i tried plugging an amplifier in that same audio jack. probably blew it up somehow. I hear it is hard to replace laptop sound cards though...ps: i have already verified that the plug is fully inserted