What is the speed record for the Boeing 747 and Airbus A380?
The published Maximum Operating Mach number (MMO) for the B747 is Mach 0.92 but it can sometimes be exceeded accidentally due to aircraft upset. It has been reported that Mach 0.99 was attained during a Boeing test flight in a slight nose down attitude.  

The A380 has a slower MMO of Mach 0.89

The actual true airspeed of a Mach number will depend on the temperature and altitude but at 35,000 feet Mach 0.92 would be about 530 Knots. (610 mph)