I know the Master System wasn't very popular in North America but anyways here (http://www.smspower.org/music/vgm/index.shtml)'s a website dedicated to Master System music.

The tracks are in .vgz format, so it's the exact original songs, which means than entire OSTs are well under 100 KB in size

You'll need a player (http://www.smspower.org/music/vgm/index.shtml) to listen to them. I use the Kb Media Player (http://download.vector.co.jp/pack/win95/art/music/player/kbmed241.exe)'s simple drag and drop feature if the Winamp plugin doesn't work.

Give special attention to the Ecco The Dolphin (http://www.smspower.org/music/vgm/unapproved/Ecco_the_Dolphin_(GG).zip) (Master System version) soundtrack which is incredible imo and *gasp* not made by the japanese.