Why do dogs tilt their heads when we talk to them?
Biologist here. Head tilting allows an animal to gain information about the vertical placement of the sound (how far up/down it is, relative to the axis of the skull). It is assumed that canids do head-tilting to try to localize a sound better. This is backed up by the fact that canids do a lot of head-tilting when hunting small prey that are hidden behind grass or snow. Generally — as bilaterally symmetrical animals, mammals already get pretty good information on left-right placement of a sound, due to the fact that we have an ear on the left and a different ear on the right — that means we can get left/right info by things like, time of arrival of the sound at each ear, and loudness of the sound in each ear. But up/down information (how high or low the sound source is) for a sound that is coming from directly in front can be difficult to figure out. This is a challenge for a predator that is typically approaching prey that are right in front. The head tilt solves this problem by offsetting the two ears vertically so that sounds from lower down will hit the lower ear first, and will also be ever so slightly louder in the lower ear, and vice versa for sounds coming from higher up. […]
With domestic dogs looking at a human, typically they already know the sound is coming from the human; they seem to just instinctively add the head tilt when hearing a puzzling sound, even if they’re pretty sure where it’s coming from.