changed print statement in callback to onCallback

was previously "performMix time consumption"
removed unnecessary whitespace
added necessary additional calls to barrier_wait, playback working

the barrier counts the main thread toward the number of threads needed
to release it. you also need a barrier_wait call at the end of each thread
in order for that thread's completion to count towards this threshold.

see here for an example: http://www.qnx.com/developers/docs/660/index.jsp?topic=%2Fcom.qnx.doc.neutrino.sys_arch%2Ftopic%2Fkernel_Barriers.html