Nope, you don't have to sign up, claim or commit to anything. It's totally no-pressure; if you listen to a song a few times and go "OMG, yes, I have a fic!" then by all means, write it and post! But I consider music to be even more subjective than most prompts, and if nothing on one particular set grabs you, then maybe the next set will be luckier!
(I'm sort of thinking about doing an extra "rush" set for V-Day--throwing out a sappy song, a bitter song, and a creepy stalker song or something--in case anyone's interested, heh, but that's beside the point...and at least a week away.)