The jet sounds the same inside the cockpit at M .98 as it does at M 1.3 . The sound is still the same relative to your ears - everything is still in the same proximity to you.
I have flown at 1.3 and what does change is the airflow around the aircraft, changing the flight characteristics of the jet. The handling of the jet becomes much more stable. If your jet could aileron roll at 360 degrees per second subsonic, it now lumbers through a roll at perhaps 120 degrees per second. Your only real indicators of going supersonic are seeing your pressure instruments (airspeed/Mach indicator and VVI) momentarily jumping as you pass M 1 and the fact that your shiny little rocket now flies like a 172, albeit considerably faster, higher and cooler.