The edition itself says that this piece is copied from the "Liber Usualis", where you can find it on p. 276. An internet search for "liber usualis salve regina" will point you to scans of the source.
The edition is thus not copyrighted, but it might be that the anonymous piece from the 11th century is copyrightd in the LU version. This is
extremely unlikely, though, especially as first editions of the LU were already published in the 19th century. In the USA, the copyright on the 1952 edition of the LU
definitely expired in 1952 + 28 = 1980, and in Germany such a copyright (on the medieval music it contains) must have expired, too, because the copyright duration of "scientific editions" or "posthume publications" is 25 years, and 1952 + 25 = 1987. Beware, however, that explanatory texts, preface etc. still might be copyrighted.