Hey, cool thread! I actually had the same thing happen to me once. Like the OP, the undies were old and worn a bit thin. I got a boner, and the front ripped wide open. Good thing I had pants on overtop! LOL. It would be worse with a speedo on the beach, unless it was a clothing optional beach!
Oh, and sorry, ripping fabric yes, but no "boing" sound effect. I suppose I have to try again now with my oldest undies, and keep a microphone handy...