I'm trying to flag a post as being relevant for Sound Design SE. As I go through the check boxes, the only site I can recommend migrating it to is Meta. I suspect that we'll see sound design questions in the future, so it might be a useful option.
I'm verifying this right now, but I believe those options are selected programmatically based on migration frequency. (Not actually true)
For now, you can just flag the post for moderator attention -- we can migrate to any site.
For that option to be added, SE staff actually need to step in and do it manually, and they may not because our site is still in beta (though our extended lifespan may indicate otherwise--and Sound.SE is certainly a valid migration path to have).