the buzzing sound can come from anything on the Arduino or stuff connected to it. I don't think that you can get rid of it just by optimizing the software - increasing sample rate will not change things. The quality out of the PWM is already superior, but get's damaged by the noise on the supply lines.Therefore as I proposed before try to use something that "isolates" the digital PWM lines from power source. Use a buffer or a level shifter. Then filter the supply for that level shifter very thouroughly using an own linear regulator or at least a serial resistor with a capacitor to gnd between VCC-digital and VCC-analog.
Regarding 4channel mode: If I translate the interrupt function to assembler then it should be possible to have enough computing power for stereo 16 bit playback...
Regarding pre-load: The function setFile stores internally something about the file in a small structure. By using that struct it should be possible to playback any file with very little latency. Maybe I will add this feature later to the library.
Hehe, I now got stereo at fullrate working - I've rewritten the audio output interrupt routine completely in ASM. Next step is heading to 4-channel audio output at 8-bit or stereo-16-bit-output...
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