As a major audiophile, and someone who loves audio tours when I visit museums and famous landmarks, I am really excited about the future of audio guides. Soon, it will seem like the Dark Ages when tourists and museum visitors had to find a special kiosk in order to check out (and often pay a pretty penny for) more in-depth information about an exhibit or architectural landmark.
The internet, mp3 players, and mp3-compatible phones with web access are changing everything. At this moment, let's say you want to go to Barcelona, and you have a phone or PDA with internet access. Or better yet, let's say you're already in Barcelona, standing in front of a famous palace or cathedral, and you want to learn more about it. All you have to do is connect to the web, choose one of the many tours available for Barcelona, for example the one produced by the Brussels-based company iaudioguide, download the tracks to your PDA, and then voila! Listen to the mp3's about the different attractions as you stand there in front of them. (BTW, I learned about iaudioguide from my friend A, who just produced their New York City guide. It has 26 tracks covering over a dozen different NYC destinations).
Demand for convenience and customization is changing market conditions for companies like Let's Go and Lonely Planet, too. Travel guide publishers (the printed kind, not audio) have realized that perhaps the 1200-page guide to the entirety of Western Europe is more than most people need, especially backpackers for whom every ounce counts. These publishers are starting to sell stand-alone chapters ready for download to a PDA or other text-friendly device.
An added advantage to both of these developments is that the user can be a bit more discreet about being a tourist. Nothing screams "Look at Me! I'm not from around here!" like brandishing a travel guidebook, map, or wand-like audio guide device with headphones. Listen to your phone, or read the screen of your PDA, though, and you'll probably fit right in!