We have a whole bunch of sounds we have pulled from all over the place. We leave standard lap times as the default beep/blip, Lap time too high is the Internet Explorer Blocked popup sound, record lap is the Southwest Airlines Ding (Standard Airline PA System Ding... you have no idea how hard that was to find!!!)
Rejected lap is a Klaxon-Awooga sound. A lot of the other sounds were already on the PC from the Scalextric RMS system... we just continued to use them with PC Lap Counter