It's ?! not !? — you're adding emphasis to a question. If it's not a question, then the sentence shouldn't have a question mark in it.
Sound effects don't generally have exclamation marks (or any other kind of punctuation) at the end. Whilst I'll use them very occasionally for comedic effect, as firm rule, I don't think they're appropriate, for two reasons…
Firstly, aesthetically, the usually-rectangular upright of the character combined with the more-or-less circular point has a habit of throwing nasty tangents on a regular basis.
Secondly, logically, the purpose of an exclamation mark in prose is to add emphasis to a phrase or word that cannot easily be denoted otherwise. In a novel, the body text has historically all tended to be the same font at the same point size with only italics, or possibly all caps, available for additional emphasis. The exclamation mark exists to add emphasis.
With a sound effect, if the text is in a poster font, bright red and set to 144pt, it's probably got enough emphasis!