I've actually been thinking about Freddy's theme song. My findings is that he has several. 

Clearly, at least one needs to be loud and angry. This would be Give it All by Rise Against. This is because Freddy puts 100% of himself into the causes he finds important(which is a lot of them). He shrieks and yells a lot, but when he's angry on behalf of someone else, "there's a reason why I sing".

Next would be Fort Minor's Remember the Name. Clearly not all of the lines apply to him(he isn't exactly quiet and humble) but several of the descriptions fit him well, and it does describe his psyche pretty well. Freddy began with the luck of meeting Sam. After that, it's been him struggling to be a good person and be worthy of his new friends(skill). Freddy deals with a serious mental disorder and refuses to stand down when his friends are in trouble(concentrated power of will). He has found some measure of happiness with these people. He even loved Puck, though the guy irritates the hell out of him(pleasure). And so, Freddy stands through his suffering, because he's got things to live for. So what if one half of him hurts and the other half hurts worse? He can deal with the pain.

His main one, however, would be this: [youtube]vYfBfDU3yhs[/youtube]

Because Freddy is angry, pissy, emotional, boisterous, brutally honest and kind of a mega douche, but god dammit he's not fighting for himself. He's fighting for his friends. He yells at them, and he stumbles and fails and messes up and is generally a substandard human being to them, and he knows this. He is grateful that they put up with him, and he loves them far more than he loves himself. They brightened his world, and helped him to grow as a person.

Plus I find it hilarious to think of Freddy ruining this beautiful song by shrieking the lyrics and adding in expletives.

"YOU INSUFFERABLE DICKWAFFLES, NOW THAT YOU'VE TOUCHED ME I NO LONGER COWER, LIKE A PUSSY, I'M WRECKING SHIT FOR YOUUUUUUUUUUU. I'M FIIIIIGHTING FOR YOOO-, GOD DAMMIT PUCK WHAT ARE YOU DOING WITH THAT HOTDOG YOU MALFORMED TROGLODYTE."