I take my cell phone with me and I load that up with 2GB of music / audio books / downloads from the BBC etc.
The music is taylored to the trip and acts as a soundtrack to the landscape. It enhances the emotional impact (e.g Benjamin Britten's Four Sea Interludes from Peter Grimes when walking the Pembrokeshire Coastal Path). There are many British composers who found inspiration from the landscape and listening to their music whilst walking within that landscape provides me with immense pleasure. It seems to connect me with the landscape in a way that reflects my own feelings and reasons for loving trail walking. I also have to admit that it allows me to indulge my romantic spirit!
I can't think of any American composers who followed the early 20th century pastoral movement that we had in the U.K with people like Vaughan Williams, Delius, Britten etc but there is a massive pool of film music that is influenced by natural environments - some of which would be great whilst walking.
Also, I really enjoy planning my trips. Exploring google earth, pawing over the Ordnance Survey maps, and compiling the music that I'm going to take with me is all part of my initial connection with the landscape before even getting there.
The other thing of course is that using the media capabilitites on my phone makes it a multi-purpose tool par excellence: Life saver, book, music player, clock, alarm, torch etc etc