I think you will find it hard to upgrade the speakers in the 8600 because I just don't think they make any good speakers for laptops. They just all suck. Get a good set of headphones or even better use an external card and set of speakers.
I just upgraded to the audigy ext sound card and a set of Z5300 and find them very good even with DVD Audio. Not audiophile, not even close to my Sonus Faber home system. But they do a good job for a computer system.