The speed of sound is a level of speed generally considered a step below supersonic speed in the OBD. But this isn't necessarily always true. Sound speed is variable depending on what it travels through, for example sound moves at 343.2 m/s in dry air at around 68°F at mach 1. This equates to traveling a kilometer in 2.9 seconds, or a mile in around five. But sound moving through water is several times faster than it is when traveling through air, (1,484 m/s) or fifteen times greater while moving through iron (5,120 m/s) And finally sound, being as it has to have something to travel through to move, has no speed in outer space.
But the OBD generally assumes that sound speed means mach 1 (for the purpose of calcs ect.) This doesn't apply to sound based attacks however, like Zebra's Missile, the laws of physics still apply to things like that.