A federated soundcloud would be awesome. If only I had more time I would finish this old project.
@dansup new photo sharing will definitely be more popularly received. Have you heard of Resonate.is? They're a co-op music platform working on a fair distribution model for artists and listeners. They just got 1 mil in seed funding and it's still going to be an uphill battle for adoption. A activitypub Mastodon/your current project would be hugely complementary. I'm excited to see how it progresses!