The Myst and Riven soundtracks (including the relatively recent digital download versions) are released under Robyn's name, so it would seem that he has ownership over the music, and Cyan has some agreement with him regarding their use in rereleases of the game. If some third party (or you) wanted to use the music, I'd imagine they/you'd have to get permission from both Robyn and Cyan, but I'd go with Robyn first. The credit information displayed on Amazon.com for both the Myst and Riven soundtracks list the Label (usually the record company that "owns" the rights to the album) simply as Robyn Miller.
If you're just talking about game design and content, then yeah the company generally owns all of that.
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Now shut up and make it so you only have to jump once. (Spoilers)
Luig'Ahn KI: (need to update)