To my understanding, there are some pieces in there copied from copyrighted work (like the Antiphonal Romanum II, for example), but is anyone aware they got the proper permission from Solesmes to copy their music?
There are various parts to consider. There's the text, the melodies, and the engraving (the score image).
They cite Solesmes books as sources (PM= Psalterium Monasticum, etc.), but it looks like they did their own engraving; I don't think they photocopied the scores from Solesmes books, so that part seems OK.
For the texts and melodies, one would have to ask in each case: is the material old enough to be in the public domain? Most of them probably appeared in older office books. If they're more recent, who owns rights for it? Texts created for the modern Liturgy of the Hours probably belong to the Vatican publishing house rather than Solesmes.
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