Well, this isn't a fix per se to the problem of the malfunctioning headphone jack, but it is a workaround which allows you to continue using your external speakers for all Mac sound output.
This fix requires obtaining a piece of hardware, namely an Airport Express with sound output. According to Apple, this device can only stream sound out from iTunes. However, with two pieces of open source software, this situation can be changed to allow all Mac sounds to be routed through the Airport Express. The two pieces of software are soundflower and the other is RaopX. Description of how to get and intstall each is available at http://www.hersson.net/projects/raopx, and as best as I can tell, they work well.
You're still left with a malfunctioning headport jack, but you have full access to the highest quality sound that you wish to expend your $$ on relative to the speakers you buy.
So, work around, but not true fix