If you play licensed music, you're on the hook for streaming royalties.
Good point. If you read the mixlr.com terms of service, they say this:
You warrant to Mixlr that:
(i) the User Content is your own original work and you own the entire right, title and interest in and to the User Content and/or you have all necessary licences, rights, consents and permissions to use, and to authorise us to use the User Content in the manner contemplated by these Terms;
Key words are "user generated content." That is the same kind of thing we post here. An example would be your own band performing their own compositions. That's user generated content. If on the other hand, you're posting and streaming songs and performances owned by record labels, then you're in trouble, and they can suspend your membership. From what I can see, they're not very active in policing their site. But once I speak with my friends as ASCAP and BMI, they may change their way.