Is it possible to fight copyright claims like this?
It's a gray area at best, and YouTube has made it abundently clear they don't care about the content creators, only about money, and lawsuits cost a lot more than censorship, so they side with the big media. In your case, Valve. The person filing the claim is a composer for Valve, and it is indeed his music. Fair Use is a difficult case to argue for "Let's Plays" as Fair Use protects " for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" Your best bet is to actually just mute the music for gameplay. A lot of YouTubers are doing that these days...
If you are actually using the music then you should acknowledge it because YouTube won't take it down unless the original owner wants it down. If you dispute it and you did use copyrighted material, you have no basis to argue.
But if you didn't use copyrighted material, then of course dispute it.
So I decided to put my google-fu to the test and found something rather interesting. The Portal Soundtrack is free for download at Think with Portals, but only the first 3 discs. At some point Valve entered into an agreement with Ipecac Recordings to produce a physical 4 disc collectors set. The 4th disc having tracks from Johnathan Coulton, The Nationals, a few other sparse tracks from the game. Ipecac has a deal with The Orchard Music Group to protect its IP. However, this track specifically is free to download so you might be able to fight against by stating that there is no monetary loss due to the track being readily available via public download from one of the original copyright holder's websites.
Ninja Edit: Ipecac also does not have the album registered in the US Copyright office so they're playing with fire now.
Well this helps out. Thanks