Err... I think the sound level is governed by the recording of the .wav file, if that's what you're referring to, Rodders.
On my PC, the sound on my sig is actually quieter than those on Sandra's and Barra's, but I don't think there's any way to adjust it from this end, once it's embedded in the .swf file.
I would have thought the easy solution would be to turn down the volume of your speakers if the sig sounds are found to be too loud.