As a filmmaker, artist, photographer - I don't like people trying to use my stuff for free, after all it's how I eat. That being said, in this digital age, there's not a lot I can do about it to stop it from happening. Is it worth my time and money to try and go after someone? Nah. However, copyright is copyright but there is caveats built into the law about how items can be used. The law is written in such a fashion that will allow use of other people's work or else the economy and knowledge of a free society would come to a grinding halt. Look into your local jurisdiction about how copyright is treated. "Fair use" might not be applicable because you're not making a parody (making fun of it) or reviewing it. My experiences with Youtube, even for legit and documented proof of music that I had licenses to use, still had issues, for a time period I had to fight copyright with every Tom Dick and Harry that came out of the wood work saying I was using their stuff. One word of advice is be careful of Sony (or was it Universal? maybe somebody else - been a while now) they may come after you. I almost had to go to court over a youtube video - they dropped the potential litigation when I showed them my licensing contract.
Will you get sued in this case? Probably not, they have bigger fish to fry. However like one of the posts above says, if you have a seriously good map and Valve wants to buy it from you - they won't because they don't want to pay for the rights to use the soundtrack in perpetuity across all mediums to the creator/corporation that owns the music. The decision is then - is this music or images so critical to the game play of the map that the map can't be played without it? If the answer is yes, then you have a problem. If the answer is no, save yourself the headache in advance and don't put them in.