Music duration

[Kurt Kaufman]

If I understand you correctly, you are trying to generate and then  
play and/or mix various music sequences using the QT musical  
synthesizers. The synthesizers can be instructed using MIDI.  I have  
worked with MIDI tracks, and what you are attempting can be done  
wherever QT is available.  MIDI tracks can be combined with a common  
starting point, or with an offset, accurate to within a tiny fraction  
of a second. You can easily play 4 (or more) tracks in this manner.   
It can appear to the user that the discrete musical sequences are  
being layered "on the fly", but actually, they are combined in a new  
MIDI file in advance of their being played.  Rev/QT can do this very  
quickly.
For more information, see my project "MIDI builder".  It used to be  
available from RunRev, but I'm not sure that it is now.  I can make it  
available if you have an interest.  The stack also includes a file  
that details the MIDI file format that I used.

Best Wishes,
Kurt