Why don't I get any sound when inserting a video as PiP?

This is the default setting. A separate PiP track offers a simple yet effective way of inserting a video clip or photo into your main video track. However, in order to avoid sound coming from both the main track and the PiP track at the same time (which would result in unclear playback), the default for the PiP has be set to no sound during playback.