Change on BPM

the audioOutput have a setTempo() function so i use it instead of the
calculated duration.
I have been able to change de duration in audioOutput.playNote() by 1
which represente a black note in the signature 4/4. In fact, by setting
the tempo of the output, the duration we have to put is the result of
the signature (4/4 = 1). http://en.wikipedia.org/wiki/Time_signature