This webpage shows you one way to do it without having to use the attic. It doesn't hide the wire completely, but at least you won't see it running up your walls to the speakers.

You basically cut two holes in the wall: one rectangular hole at the point where you want to mount the speaker, and another small round one (for the speaker wire) directly below it near the baseboard.

Completely hiding the speaker wire in the wall is a bit more involved.