There are two sources of the Music Device Configuration software required to switch from the Macintosh Built In Software Synthesizers and Modem output to other MIDI hardware equipment.
The first source is a CD-ROM bundled with the book - "QuickTime - The Official Guide for Macintosh Users" - by Judith Stern and Robert Lettieri from Hayden books. $45.00
The second source is the QuickTime 2.0 Software Developer Kit available from APDA. $195.00.