Well, I'm not clear whether the thread is about songs that you listen to (big yawn) or songs that you sing out loud when the crunch is on (now we're talking).

It may be impolitic in present company, but the song that has got me up many a steep, slippery hill with full pack (and other equivalent situations) is "Barrett's Privateers" by Stan Rogers.

Though sometimes, I have been known to simply utter "B@stards! B@stards! B@stards!" over and over, kind of like a mantra, to keep me the noradrenaline flowing and me moving forward.