Normally, air gets cooler with altitude and this is what we expect to experience. We see snow at the tops of mountains, but none in the valleys. During a temperature inversion, warmer air sits on top of cooler air, creating a sandwich of cold air near the ground.
Sounds get trapped in this sandwich layer, meaning we get to hear things that we can’t normally. In this video we can hear the normal nearby sounds (dogs stepping on leaves), but also distant birds and roads that can’t normally be heard.
And it is not just sounds, temperature inversions also trap smells.