Quite a few of the carols from Piae Cantiones are in regular use in modern English language hymn books and carol services today, for which we have to thank John Mason Neale and Thomas Helmore. Neale translated many of the texts from the latin, wrote new words in some cases, or in at least one case (Corde natus ex parentis) set much older words to the music. The original poster is right of course that many of these tunes are much older than Piae Cantiones, In Dulci Jubilo certainly is.