Woah. Where did you get a batarang sound without the music? The only episode I can really think of with that is Blind As A Bat.
Also for the activation, a neat sound would be the sound they use when he fires the grapple gun. The cable sound could even be used as the hum. It might be harder to find those without music though, unless you can filter out music. I never did figure out how to do that without it sounding weird.
Now we just need a Mister Freeze cold gun font.
I never thought of using the grappling hook firing as an option, it sounds like a really good idea. Might have to play around with it again.
As far as the Batarang, yeah, I got it from the episode "Blind As A Bat".