Just wanted to thank you again for such great library. 8)
I managed to find the buzzing sound, it was my wiring, a friend told me to not use too long wires to connect to the SD Card, that was the reason of why the SPI Clock was leaking to the PWM outputs. Its solved now.
I still wonder if it would be possible to handle WAV files (saved at the right sample-rate) but its not a big deal to convert things up, just wondering. (one less step for newbies)
The 16-bit mono support is great, but yes, stereo support would put this lib in a new position.
I plan on using 4 loopers running side-by-side, already got extra ATmega328 + Crystals here to do so. 
Just need some extra time, prob next week...