I have no idea what you are saying about multiple sounds. If you've literally tried playing a sound more than once to make it louder, then lol.
Just use volumescale. You can use values above 100 for volumescale in an SCTRL. You can also use volume and/or volumescale under the [Data] section in the character constant file and that number can be above normal.
[State 1500, striker shoe]
type = PlaySnd
trigger1 = AnimElem = 5
Value = s1100,0
volumescale = 250 ;; seriously.
channel = 3