Seattle Airsoft has a great post on measuring BB velocity using a microphone. They use two pieces of paper separated by 10 feet with a microphone by each one. Audacity is their program of choice for recording. If you look at the waveform after firing you’ll see three distinct spikes: one from the firing mechanism and one from each paper strike. They compared the results from this method to those of an actual optical chronograph. The mic seemed to deviate far less.