Super 8 film stock.

[Agulliver]

I would imagine with modern laser technology and miniaturised electronics, super 8 optical sound might be more possible than in the 1970s when magnetic tape was basically your only choice. But nobody will be foolish enough to sink their fortune into such a system.

I liked shooting super 8 sound cartridges but that's been a pleasure lost to me since 2004 when I shot my last expired 200 foot cart.

Martin Baumgarten, who has more recently retired from selling cine film, was looking at striping film and loading it into sound cartridges circa 2000 but nothing came of it.

 

[Harry Callahan]

Yes, especially because now there is more fine-grained film stock. With optical sound sound quality also does depend on size of the grain - the smaller the grain the better the sound.
But today, as optical sound could be better than ever, no-one is interested any more.
On the other hand magnetic sound always was better than optical sound, so even if a millionaire starts to have a soft spot for optical sound on S8/Single-8 still the question was whether it would pay off in comparison to magnetic sound.
On the other hand optical sound on the film would never fade and it was cheaper as you don`t have to pre-stripe the film...

... i never shot with a sound camera. I once tried to do a sound movie (on the pre-striped Single-8 stock) with a portable tape-recorder (no sync), but i didn`t got the movie finished. It must have been great to have an all-in-one-system with lip-sync sound.