do you think a music streaming site or digital downloads area would benefit Steemit?

Absolutely. It would benefit Steemit, Steem, and STEEM...but most importantly - it would benefit the musicians.

The music industry is huge and plenty of musicians maintain blogs, so this would likely catch on pretty quickly just from the interaction/income angle for all of these "starving artists." Building an actual marketplace where they can sell their songs, without having to go through a third party (labels, publishers, master recordings). Cutting out losses from fees and whatnot would be a boon for many artists, especially those who have already gained some popularity.

The ability to do this would likely attract a large chunk of musicians...and even songwriters looking for artists, and vice-versa. You could create an internal marketplace for composers and performers comprised of your users, possibly with smart contracts integrated.

On the consumer side, this would probably reduce costs per song since the artists don't have all of those contracted fees to pay and since they'll also have the opportunity to earn from their followers on the blogging side of the equation. And...it's music. I'm sure plenty of Steem users would love to be able to use their STEEM and SBDs to purchase it. This keeps the currency in the Steemcosystem, which is kind of the point of all of this anyway, right?

I would like to help you develop this idea. I unfortunately don't have any coding experience, but I can offer other assistance. Just let me know. You know where to find me!