Steam through the Early Access program sometime in September. The news comes from a brief blurb over on the game's official site.
Previously, the sequel was billed as Audiosurf Air. Creator Dylan Fitterer's studio, Invisible Handlebar, even went so far as to launch a website and solicit beta applicants for Audiosurf Air early last year.
Predecessor Audiosurf - a game that builds custom 3D levels from your music library, which you then "surf" in the game - first launched on Steam in February 2008. Audiosurf is often credited as being a pioneer in the modern indie games movement.