im not 100% sure but i think with the sound thing, ull only get an accurate reading if you air up ur gun then do it.. otherwise the sound software could be picking up other sounds, like pulling/releasing the trigger count as two spikes.. stuff like that. but i dont know for sure, try download.com and search for a sound thing.
download a sound recorded that has the wavy lines that show when you make a lound noise. shoot your gun while recording, then look at the wavy lines to see how many "peaks" you get between the seconds