In "Ye Sacred Muses," Byrd bemoans the death of his mentor, Thomas Tallis.I like to think of this piece analogously with Monteverdi and "Lamento d'Arianna" (see my post about Monteverdi and Mozart):
Byrd : "Ye Sacred Muses" :: Monteverdi : "Lamento d'Arianna"
Both composers lost a loved one (Byrd a mentor, Monteverdi a wife).
The non-ecclesiastical nature of this music indicates that this piece was written before the reign of James I, for Byrd almost exclusively composed Church music after Elizabeth I's death. Indeed, Elizabeth's protestant ideology and simultaneous respect for Byrd's work essentially forced him to innovate and explore his own artistic style.
Next up is a portion of one of Byrd's masses (written after the coronation of James I) performed while an altar is prepared. Byrd wrote several masses for both Mass Ordinary and Mass Proper. Enjoy!