Silly layman question: where is the actual sound generation done? Because I don't see the Raspberry Pi as free of electrical noise...

(Still have memories of trying to find the usb port that didn't introduce noise in my usb creative thingy...)

In the case of both Zynthian and Monome systems, both based around the raspberry Pi, there is a dedicated ADDAC circuit added to the case.

For things like the Korg synths which use the raspberry pi, they added their own DAC's too.