When you here -nothing- while trying to play back MIDI files using an SBLive! soundcard over "A: SBLive! MIDI Synth" or "B:..", you might have removed the default soundfont library loaded into you soundcard.
(that's what I did... )
Another thing that can happen while playing around with SoundFonts is that you delete your general set and only load soundfonts for a single instrument (e.g.: Drums) - Then you hear the drums playing, but there's nothing else.
[SOLUTION]
Restore the default soundfont situation by loading one of the original soundfonts that came with your SBLive!. They're called:
...and should be on your SBLive driver CD in a folder like "/audio/common/sfbank"2gmgsmt.sf2, 4gmgsmt.sf2, 8mbgmsfx.sf2
For everyone out there who can't find that CD anymore, here are download links:
http://www.mudz.org/download/Drivers/SB ... on/sfbank/
http://www.das-werkstatt.com/useful/sfbanks