I checked the scripting reference and also a couple of book on Lingo and I didn't find anything... Maybe this can't be done. I know sound channels go from 0 to 255, but that's not what I want.
You can't set 'the soundLevel' to anything other than an integer between 0 and 7. You can set the 'volume' property of specific sprites and sound channels to values between 0 and 255. You could fake 14 steps using the volume property - the soundLevel will only provide 7.