Usually, as a first test for audio event detection, I use YouTube as a source for various audible events. But the real-life test is irreplaceable. In the case of PionEar sensor, it was a bit tricky to catch the ambulance with sirens on and have a camera ready. Last time I spent 2 hours driving around the nearest hospital and wasn't able to meet any car with sirens :-) Today, on my way to work I was lucky and made a shot.