In theory, ground speed can be higher than the speed of sound if there's enough tail wind. Not sure if it ever happens in practice?
Quite common on translatlantic routes.
Cruise on a 747-400 is about mach0.85 (925km/h) and you commonly have a jet stream of 150km/h so a ground speed of close to 1100km/h is possible - above the speed of sound at 30,000ft
Locally, at some point over fuselage/wing airflow is bound to be supersonic. ;-)
I learned this in my Aero Engineering courses in college. My professor mentioned that occasionally if the light hits the wing at the right angle, you can actually see the shockwaves.
But for all my years of flying since, I've yet to see it... :(
Under the right conditions you can expect to see some puffs of condensation due to the adiabatic cooling of the air on the edges of the shockwave. But then only under the exact right circumstances (moist air, sun from behing the viewpoint, relative high pressure).