A website setup with passionate music fans that do some writing. Then the entire iTunes catalog is on the site via their affiliate program (which is either via CJ or LinkShare). Everytime someone buys a song via a link on your site from iTunes, you receive $0.05 (or 5%) of the sale price.
You could setup the website so that all affiliate commissions go directly to one charity — or the user could select from a few charities that you have preselected. (Users could also suggest future charities).
Hopefully everyone would take the extra minute to then purchase the song via your website, rather that direct from iTunes — as to support these charities.
You could also do affiliate sales of music t-shirts, ipods, and everything else music related.
Another site that does this but not focused on a particular niche is iGive — they give a % of each purchase you make via their website to charities.
It’s just like when your school or organization got together to do a fundraiser — people all over the world that have various passions (niches) can collaborate to help raise money for others. It’s all about finding the communities online — they will work together to help make this world a better place, they just need the (web-based) tools to do so (including widgets).