Just realized that it's not really documented anywhere that your purchase comes with the game's soundtrack and the option to subscribe to a monthly newsletter we send out to talk about our design & development process.
The soundtrack mp3 files come with the game when you install it via Steam. If you don't know where this folder is on your harddrive, an easy way to find it is to right-click on the "Kentucky Route Zero" listing in your Steam library, choose "properties" from the drop-down, switch to the "Local Files" tab in the window that opens up, and then click the "browse local files" button. This will open the folder in explorer (or finder/nautilus/dolphin/konqueror/etc.)
The soundtrack for act one is in a folder called "Act_One_Soundtrack". As we release the remaining four acts, you'll get folders containing their soundtracks as well. If you'd like to share this music with your friends who don't own the game, we suggest sending them a link to Ben Babbitt's page on Bandcamp, where folks can listen to the music free and purchase mp3s: http://benbabbitt.bandcamp.com/album/kentucky-route-zero-act-one
Also in the game's folder is a file called "subscribe_to_mailing_list.txt". This contains instructions to subscribe to the game's mailing list, if you're interested.
You'll also find the game's README and release notes here, which contain some common troubleshooting tips.