Figured it out!!!! From a Mixmeister forum:
a solution :
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********* Dual Audio Output Configuration in Snow Leopard *********
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1) in 'Audio MIDI Setup' create an 'Aggregate Device'
with 'Built-in Output' for 'output 1 & 2'
and an 'USB audio card' for 'output 3 & 4'
2) under this new 'Aggregate Device' select and
set the 'Built-in Output' and the 'USB audio card'
with parameters '44100.0 Hz' and '2ch-16bit'
3) in 'System Preferences' set the volumes of
'Built-in Output' and 'USB audio card' to the maximum
and choose 'Aggregate Device' for system output
4) in 'MixMeister' choose 'Tools > Audio Output Configuration'
and select the 'Output mode' option to 'Dual output' with
'Built-in Output' for the 'Main output (speakers)' and
'Aggregate 3 & 4' for the 'Preview output (headphones)'
! before hearing in the headphone set the volume !
! output at the end of the timeline to -25 !
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*** if futures executions of MixMeister 'Dual Audio Output' ***
*** has no sound go to 'Tools > Audio Output Configuration' ***
*** and select the 'Output mode' option to 'Single output' then ***
*** to 'Dual output' to re-initialize the audio configuration ***
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Kerry
http://clubkerry.podbean.com