Here’s what I’ve found to be useful. First, start with the intention to have a mindful/meditative walk – walking a bit slower than a normal walk/hike. Second, commit to being silent so that it’s a full-on sensory walk without divided attention. And, third, focus on your breath (in-breath, out-breath) so as to stay present in the here-now. My preference in for a circular breath – in through the nose, and out through the mouth – which keeps me focused on the physicality of it all.
With each in-breath, for sounds, visualize the sound coming in through your ears in the same way that your breath is coming in through your nose/mouth. On each inhale, choose one sound to focus on. You can choose a sound that’s closest/farthest, loudest/softest, unusual/usual, natural/manmade, etc. – just don’t judge (no sound is better than another, only different). Release your focus on the exhale, and pick another (or the same) sound for the next inhale.
Photos - Creative Commons (Kara Shallenberg "Ear;" Mike Lindsey "Eye Spy"