Here is a bit more information.

First check out this primer on the Quicktime DLS player:

http://testtone.com/developers/apple/dls-music-device/review

So the interesting thing is that on the Mac you can have many banks of sounds available to load into the Quicktime DLS player, but you can also configure it to stream from disk rather than loading the whole thing into memory first. When the quicktime DLS player is used from say, garageband, or just about any DAW will recognize it as one of the AU plugins and you will see these options to choose an SF2 soundfont bank and optionally to stream from disk.

Unfortunately BIAB does not provide that interface currently.

However there is still one more way to override the sounds and use them with BIAB. Quicktime can play both soundfont SF2 files and DLS files. The default soundbank used by quicktime is a rather small 2MB GM soundbank (no wonder it sounds cheap), its in DLS format and located here:

Code:
  /System/Library/Components/CoreAudio.component/Contents/Resources/gs_instruments.dls


You can convert Soundfont banks into DLS banks using a Windows program called Extreme Sample Converter. Unfortunately that program is not free. I do not know if there is another way to convert SF2->DLS maybe so. In any case, if you convert a GM SF2 bank into a DLS bank and move it into that location, then that will be loaded by default by any program that is using the quicktime DLS player, including BIAB. However, since you can't configure streaming, it may take a long time to load. I found that this worked with BIAB, but for a 60MB soundbank (which sounded a lot better by the way), it took a couple of minutes for BIAB to start up, presumably because it was trying to load this 60MB DLS file into memory somehow and it took a while. It did seem to play fine.

If PGMusic would consider improving their support for the DLS player, to choose banks and to select the streaming option, it would open a lot of possibilities. There are many SF2 GM and GS soundbanks out there, they are used a lot in the karaoke world.