The millis() is used, and with those calculations the sine value of that moment is calucalated. I don't understand all those calculations, but I can see the 256 and the 60 in the code.
I think that you can not use those functions.
The analogWrite() has a PWM frequency of about 490Hz (some pins 980Hz):
https://www.arduino.cc/reference/en/language/functions/analog-io/analogwrite/
With 490Hz, a frequency can be made of 20Hz, but then it has distortion.
I suggest to use a board that has Sound or DSP or a library for Audio.
For you Mega board, there is Mozzi, but it has only one audio output.