Hey! You can lock the pitch to a scale using the buttons on the right side of the Pitch knob. Engaging the button at the bottom for example will snap the value of the pitch knob to the nearest octave.
You can then assign modulator to the pitch knob to generate melodies (or chords, depending on the Length and Rate of the grain generation)
Stream 1 does something like that in the “LFO Arp” preset. It should sound even better with just a single note frozen in the buffer
Hope this helps