I'm not entirely clear on which sequence of events you need, but if the intent is for the first press to lock, then the second press fires and resets to requiring a lock:
Lock/Fire
Begin Boolean Compare : [MissileLock] Does Not Equal True
Set Boolean [MissileLock] to True
Play sound, '{VA_SOUNDS}\hcspack-ELI\Targeting\Missile lock.wav'
Else
Set Boolean [MissileLock] to False
Play sound, '{VA_SOUNDS}\hcspack-ELI\Weapons\Fire missile.wav'
End Condition
If you instead want the first press to lock, then each subsequent press to fire, with a separate command to reset to requiring a lock:
Lock/Fire
Begin Boolean Compare : [MissileLock] Does Not Equal True
Set Boolean [MissileLock] to True
Play sound, '{VA_SOUNDS}\hcspack-ELI\Targeting\Missile lock.wav'
Else
Play sound, '{VA_SOUNDS}\hcspack-ELI\Weapons\Fire missile.wav'
End Condition
Reset lock
Set Boolean [MissileLock] to False