It seems the sound played when Cavalry Attack is used comes from the special effect Abilities\Spells\Human\FlakCannons\FlakTarget.mdl. This special effect is used to create the dust when units are moving, but it automatically plays a sound (that sound is attached to the special effect model). What you can do is replace the sound FlakCannon.wav (located in Abilities\Spells\Human\FlakCannons\) by using the same procedure as Warseeker told you about (in sound editor, right-click on the sound file and hit "Replace Internal Sound").
If you do this, be aware that that sound will be played every time FlakCannons special effect is used: therefore, when a Flying Machine uses it, it will play your custom sound instead of the regular sound. It is possible to bypass that issue, if you really need the Flak Cannons ability: you may edit the model FlakTarget.mdl and change its sound, then import it again and use it either for your custom ability or for the Flak Cannons ability.