Rafter trusses ARE typically 2x3 on 24.

Stick construction can be 2x6, 2x8, 2x10, or even 2x12 depending on the span, so each case will be different.

I've done pm ceiling and in wall speakers much the same as I have with recessed lights in my own home. Used rigid foam to make an enclosure. Seems to work about the same as speaker boxes in that there's some soft surface inside the box that somewhat dampens reflections.