Simple 3 audio channel mixer

hi all i was testing the example included in bassdll library http://blog.makezine.com/archive/2010/01/bassdll_a_sound_library_for_arduino.html. basically you attach three piezo, one for pin 9-10-11 each, and arduino plays via pwm a nice 3channel tune. to mix the three signal into one little speaker, i used this raw circuit: the two op amp are inside a LF347 which is alimented externally by a 12V 500ma max wall power supply. In teory the sound should be loud and clear, but what i got is nothing (except the 50hz noise - btw, how can i get rid of it?) so, any hint?