Rax has a recording feature. So you could record the MIDI playback as audio. But in my experience Rax is a bit unreliable. I've got several crashes while experimenting with it.

Personally, I prefer using Logic Express in conjunction with BiaB. As a full featured DAW it can be used for playback, recording and editing of MIDI and audio.

The setup of Logic as a multi-timbral sound module (with recording facility) is described here
http://www.logicprohelp.com/viewtopic.php?t=32201