you need to use a Digital to analogue converter to return the PWM signal to a smooth wave and then amplify it to audible quality.
As I understand it you can't simply use an arduino to output audio as it's processor isn't good enough and likewise it's PWM doesn't pulse fast enough.
Is an I2C DAC too slow,
Parallel ones are not good with the arduino due to lack of a whole port with all the pins free. However it is fine on a Mega.
Hack the library and run it at 400KHz
Port B is free
why do you need to have the whole port?
#ifndef TWI_FREQ #define TWI_FREQ 100000L #endif
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