http://www.jamkazam.com - we are not yet VC-backed, but I hope so soon.

Because latency is an absolute premium for playing music in real-time across the internet, we've focused first on the desktop, where we have the most control ... but still find a ton of challenges.

If you saw this recent article about Android and audio latency, you can see why there are challenges in the mobile space: https://news.ycombinator.com/item?id=9386994

If you were to try and build a web-only version of JamKazam, WebRTC is your best bet but it does not have low enough latency, either.