or an Exec Function? Like:
Not sure if this would work, but if it did you would have to bind ChangeGroundSpeed to a key and everytime you pressed the binded key your speed would increase by 10. To bind type in console: "set input (key) ChangeGroundSpeed".
class coolpawn extends xpawn;
var() sound ChangeGroundSpeedSound;
Exec Function ChangeGroundSpeed()
PlaySound(ChangeGroundSpeedSound, SLOT_Misc, TransientSoundVolume,,TransientSoundRadius,, false);