According to my copies of the DVDs (a 1999 DVD of the film and the 2007 Collector's Edition), the copyright in the film is owned by Swan Song whereas the copyright in the bonus materials (deleted scenes, omitted songs, etc.) is owned by Warner Bros. This suggests that Page et al. own and control only the original film and probably only the soundtrack to that original film and that Warner owns whatever was left over and not used in the film.
My original vinyl (purchased new many many years ago) of the "soundtrack" says the copyright was owned by WEA Records. Ltd. Assuming WEA is basically Page et al., this is consistent with ownership of the film and the soundtrack to that film by Page et al.
My copy of the 2007 CD remaster of the audio "soundtrack" album indicates copyright ownership by both Atlantic Recording Corp. (identified as a Warner Music Group company) and an entity called "Mythgem, Ltd." Because there are extra tracks on this edition (e.g., OTHAFA, MMH, etc.) the copyright ownership information here is consistent with ownership of the film and the soundtrack to that film by Page et al. (assuming Mythgem is basically Page et al.) and ownership of the left-overs and unused stuff by Warner.
So, in summary: Copyright notices indicate that Page controls the original film and the soundtrack to that original film and that Warner owns everything else associated with the film (including audio tracks).
My best guess is that the revenue-sharing agreement for the audio album, between Warner (Atlantic) and Page et al. (Mythgem), splits right at the line between "soundtrack" to the film and "live performances" recorded in connection with the film. Thus, to ensure maximum value of the copyright in the audio album (and therefore the maximum revenue share from sales of the album), Page might have had an incentive to ensure that there was no dispute that the bulk of the audio album was Page-owned "soundtrack" versus Warner-owned "live performance." In theory, that might translate to editing some songs to ensure that they are viewed as "soundtrack" to the film under current copyright law and practice (which may have changed a bit since 1976).