Actually, SRS, my explanation requires neither mispronouncing nor misunderstanding, just misspelling. If Mr Pole sang "jubil hounds" to mean "joyful hounds," this appears to have been a real word used correctly, and is in the OED with that exact meaning. Remember that other texts of the song do in fact give "joyful hounds" so all the speculation on whether "joyful" itself can be right given that the song is from the fox's point of view is a red herring. The song does call them "joyful" in other versions of the same line.

The only mistake made was that RVW wrote it down as "jubal," which in all the versions I have heard (which I grant are from revivalists, not Mr. Pole hmself) is pronounced exactly the same. So he just spelled it wrong; he may have understood it, though.