I don't quite get it when you say you have access to files, but they execute locally... I wouldn't expect installed programs to function across the machines, unless it's network "knowledgeable" application - still means you probably have to install part of it on the "second" machine... You could execute stuff on one machine and have it visible on the other - VNC, some of the remote admin/hacking/cracking tools...

But to simply be able to play mp3 stored on one machine on the other, simple file sharing should do - just "Map network drive" on the machine where you want to play the mp3 and point out the proper drive (or folder) on the other machine and it'll turn up as a drive letter.

/Michael