It's completely depends on the DAC you get. It's usually pretty simple, you simply output the data to be written on the DAC's data lines and toggle a strobe line to set the DAC's output. Some DAC's have buffers and can even be used to read the current output state, but again it's totally dependent on the exact DAC chip you pick. You can get passable audio from a PWM DAC too, which is simply running a PWM output as fast as you can (over say 40khz) and modulated the duty cycle to match your audio data, then you use a low pass filter (resistor and capacitor) to smooth the output a little. You should be able to find plenty of sample code using Google to search for PIC audio projects, PIC's are extremely well documented on the net.