Why are swept wings better for breaking the sound barrier?
It's all about the 'effective' Mach number that the wing sees.  In essence, for a swept wing, the Mach number that the wing 'sees' as far as compressibility effects are concerned is the Mach number of the component of the freestream normal to the wing chord:


(Source: stanford.edu via archive.org)

By sweeping the wing back, you can get closer to the speed of sound without getting supersonic flow over the wing (remember, a wing generates lift by accelerating the air flowing over it, thereby decreasing the pressure - so you'll get supersonic flow over the wing before the freestream is supersonic).  Of course, once you fly supersonic, there's really no getting around having shock waves over the wing - however, you can reduce their strength with wing sweep.