Hi there,

What you're essentially looking for is a VST instrument plug-in or a SoundFont that implements the General MIDI standard and replaces the crappy Microsoft GS Wavetable Synth. There are many different options here, including a bunch of rather pricey commercial solutions. If you're looking for a quick and free solution, you should probably download and install a  simple SoundFont/sample player as your VST plug-in and load a suitable GM SoundFont.

Use SFZ as your SoundFont player and integrate it into Music Maker as a VSTi:

http://www.cakewalk.com/support/project5/sfz.aspx

Next, download some SoundFont/SF2 files and load them up in SFZ. Here are a few options:

http://www.geocities.jp/shansoundfont
http://soundfonts.homemusician.net/collections_soundfonts/musica_theoria_v2_gm.html

Here's a big collection of SoundFonts:

http://hammersound.com/cgi-bin/soundlink.pl

Check under the "Collections" section to find some complete GM instrument sets. Look for the "Fluid Release 3" (currently on page 2). That one is pretty good. You'll have to uncompress it again using sfArk before you can load it into SFZ, though.

I hope this helps!