Nope, that's not IAS, but ground speed. IAS is indeed indicated airspeed based on your present lift in standard conditions (sea level, temperature etc.) and does not exceed 340 knots. True airspeed is IAS corrected by the altitude and temperature. Ground speed is true airspeed + wind. In order to see sonic booms your true airspeed (not the displayed ground speed) would need to exceed mach number. On high altitudes, the upper speed limit is bounded primarily by mach number (ratio of your speed to speed of sound), because a conventional aircraft would stall (high speed stall) if they come too close to the speed of sound.