You're not hearing all the instruments because of Vista's sound system.
You probably noticed that instruments are missing when you use "Microsoft Synthesizer" as a music device..which you have to use in order to get any good sound at all and for Samuel's DLS.
Soooo...to the point,you can fix it!
You have to place an "older" dmsynth.dll in the folder where your game is installed and that's it.
I uploaded the file
here.I have Vista 64bit and it works great.
An easy way to test it is with FF8MIDI from the main site
www.qhimm.comWhen you apply Samuel's DLS files and try to play "Balamb garden" with "Microsoft Synthesizer" selected as the device I guess you won't hear any sound at all for like 12 seconds.
If you put the dmsynth.dll file in the same folder as FF8MIDI and then start it you should now hear the sound as it should be.
So...once again,dmsynth.dll has to be in the same folder as the main game .exe or any other program/game you intend to use it with.
edit:oh, X-Fi ...well...I never had an external sound card but try this dll stuff,I hope it will help
As for the main topic question it would be really nice to exchange the sound with mp3s
... ...
edit: may 2011.I'm not entirely sure, but I think Windows 7 got an update which fixed the overall broken midi support in it (which was done for Vista SP2 but not for Win7 for some time after it's release).
So an updated Windows 7 or Vista SP2 shouldn't run into any problems with Samuels DLS.
If for any other reason you're running Vista/Vista SP1 then just put
this dmsynth.dll to where your FF8.exe is.