So there have been a lot of contenders to replace JavaScript, and it hasn't budged from it's market seat at all. Here's why:
You can do almost anything in JavaScript these days. The down side? You can do almost ~anything~ in JS these days. Including lots of stupid things.
Since JS works extremely well for user interaction, and is "good enough" for simple object-oriented programs, it will likely never go away in the foreseeable future in these realms. What I think things like SoundScript are for, is 1) for people who are used to strongly-typed programming languages to feel more comfortable on the web (which is actually pretty important), and 2) for larger OO or enterprise-style systems that really shouldn't have been written in JS in the first place (I'm looking at you Angular).
I wonder if it will end up like Go, though. An interesting language which never gets more than a niche market.