What is max freq. for sound system datafetch ?

Max sample frequency is around 30KHz - this is because the sound DMA has access to the system RAM only at the start of each scanline, and each of the four channels pre-fetches 2 samples at that point.

What method is used for sound output ?

A simple Delta-Sigma converter with an RC filter (two of them, one for left and right).
Running on 16MHz.

What is quality of sound output, if compared with Amiga ?

It's about the same.