K8118 Stereo Ultrasonic and Bat Detector - Interference on left channel

I have built the kit, but on testing the left-hand channel seems to be picking up a lot of what seems to be radio interference and the mic has little response. The right-hand channel works well. I have swapped the two identical chips around without effect. I am an experienced solderer, and my soldering is clean and neat.

Can someone plese give me some advice on what to do next.