The music was recorded by Seeburg's own orchestra, using songs either in the public domain or for which they had purchased unlimited rights. Just like the modern "ghost artists". So this business model goes back to the 1950s.
The records had a form of copy protection - nonstandard RPM, nonstandard size, nonstandard hole size, nonstandard groove width. So they didn't file copyrights on all this material. As a result, there are sites on the web streaming old Seeburg 1000 content.
Seeburg made jukeboxes with random access, but the background player was simpler - it just played a big stack of records over and over. It's rather low-fi, because the records were 16 2/3 RPM, which limits frequency response.