Why is some software not appearing in the "available software" options?

I have installed and used some applications such as Quod Libet and others on Ubuntu 12.04. When I go under settings to tell Ubuntu which software to use when opening specific files, Quod Libet and others are not in the selection. Even when I click "Show Other Applications" it is not listed as one I can use. Now if I go through Quod Libet, I can tell it to play that file just fine so I know it will play, for instance .flac files, but why is it not showing up so I can select it as my default music client?

Thanks and apologies if this is already in another thread. I wasn't sure how to phrase the question.