Don't know about the serial key, but you can unzip the zip file containing the music with 7zip:
http://www.7-zip.org/download.html
After installing 7zip just right-click on the mp3 zip file, or right-click and drag, and select the 7-zip option

No idea what the keys are for, but music is just for you if you would like to listen to it. The game is complete without this music in zip.

Edit: key code is probably for creating your own prisoner that will be added to the game.
http://www.gog.com/game/prison_architect_name_in_game_dlc - view description

Usually for multiplayer.

On the ZIP file, I know how to unzip it. I was asking more about where I should unzip it so that the music files it contains will actually get used by the Prison Architect game.

You can 'join' Prison Architect and I'm pretty sure that's what the key is for. Go to their web page, I'll dig up the address here in a few minutes, and I think you use the key to register at which point you can do a bunch of stupid stuff, and one really cool thing: enter the system! :D You get to make your own prisoner and next update they have they put him in the game. You can find me in there as Bram "Tiny E" Toker.

BBIAB with that address.

Here is the address.

Don't worry, it's not like joining Steam. There is nothing to download, no personal info, and they won't spam the shit out of you.

http://www.introversion.co.uk/prisonarchitect/register/

No need, the game has the music. The seperate zip is so you can listen to it. Like owning the soundtrack to a movie.

The soundtracks you can download on the game page in your account are extras, for your separate listening pleasure :) The game doesn't need you to do anything with them

I was hoping the ZIP containing music files I purchased as an extra with the game Prison Architect would actually be used by the game for its sound track. Will the game use these music files? Do I simply unzip them in the top directory? There were no instructions or readme file in the ZIP.

The game should already have the music in it and the zip should ONLY be meant for separate music players. ("Should", in both cases, because I haven't actually bought the game yet, but that's how every game I have works.)

The game already has all the music in that zip inlcuded in the game. The zip file is only used if you want to listen to that music while the game is not on.

Those are the soundtracks of the game for your own use. So you can for example copy them on your phone and listen to them if you want. The game's installer contains everything the game needs to run.

Thanks everyone for your helpful responses!

Sorry I'm confused. Are you saying you purchased music as an extra? Are you referring to aficionado? In that case the extra music is probably only that, an extra for separate listening pleasure.