Yes, it is possible to a r/c airplane brake the sound barrier. and you even didn´t need a exotic turbine or rockets to do it.

Just tie a sleek and strong model to a high altitude balloon, with a release mechanism and some
kind of optic aid, like a long range onboard video camera. Release it from high altitude and then
you would see how many mach´s you reach in function of the altitude of the release..

Best regards, Enrique