It sounds like an unusual prospect, but Jean-Luc Pontico has created a convincing Sound RTS. It's cross platform (Windows, Linux, Mac) and localised (French, English, German, Italian, Spanish) and free!
I'm particularly impressed with the menu system and the speech samples used. Very clear and easy to use. Real time 3D audio works reasonably well to give the impression of units moving around, and continual reports of building progress are useful and analogous to visually observing the state of a build or progress bar.
Apparently it has mutliplayer features and allows the players to use their own maps as well. There's an active forum and blog.
This is an impressive game with high quality production values and a complete RTS implementation.