I'm not sure you read the article. "Soundscript" is just a fancy name for "notifying the Javascript interpreter that you promise never to do certain things, thus the interpreter can stop checking for them on every access and the optimizer is allowed to make stronger assumptions". It's not a new language so much as slightly different 100%-backwards-compatible dialect.

It is a way of deliberately putting down some of the often-silly dynamism Javascript provides.

Whether it takes off or not, I can't call it a "contender to replace Javascript" when it is Javascript.