Dogs are man's best friend for a reason, and part of our connection with them comes from their emotional intelligence being off the charts. One suggestion of why they tilt their heads is that they're reading our emotions and looking for the positive response they usually get when they do it. (That's right, your dog is trying to make you smile. Granted, it might be in hopes of getting out of trouble, but kudos for ingenuity.)
There's a couple other theories, too. Depending on the shape of your dog's head, they may be able to improve their ability to pinpoint the location of a noise if they adjust the position of their ears, clearing their line of sight at the same time. Studies have shown that dogs with longer muzzles tend to do this more, so it's likely there's some kind of physical, sensory benefit they're getting from it. Hold your fist in front of your face to mimic a dog's nose, then move your head to see what sort of difference it makes in your line of sight. Go ahead, we'll wait.
Yet another theory suggests that they're just concentrating particularly hard, trying to read what you're communicating to them and seeing what words they recognize. They're looking and listening for social cues to figure how they're going to react, and waiting to see if you're going to say something good (Ride?! Treat?!) or something bad. Experiments on dogs' interpretation of language found that they'd tilt their heads right when they were spoken to with emotionless words, and left when the words were full of emotion, both good and bad. That seemed to suggest it has something to do with the way they process information, and it might mean they have separate processing centers for emotion and words.