Maybe the hounds came from Juberlane...Or maybe they were used in hunting rodents and really were "Gerbil hounds"

Seriously, I mostly agree with Malcolm, except I suspect even more strongly that Mr. Pole had the Latin root "Jubile" in mind. Indeed, I think ClaireBear was the closest of anyone!

Looking in the OED, you will find the word Jubile. Although this is a variant form of "Jubilee" and usually pronounced as three syllables, the OED specifies that it is sometimes pronounced as two syllables. Further, it is sometimes used as a prefix to denote "Joyful" or "celebratory." The example they give is "jubil-trumpet." (All of this is in the entry for Jubilee).

Note that the second vowel sound is not accented and thus pronounced as what phoneticists call a "schwa" (that's the upside-down e in the phonetic alphabet). In other words, the "jubil" of jubil-trumpet is pronounced exactly the same as the "jubal" of "jubal hounds."

Given this, it is conceivable that

1) jubil (meaning joyful) existed as an obscure dialect word until Mr. Pole's day, and it was an everyday word to him.

2) Mr Pole (or someone in the chain of transmission from which he received the song) was aware of such formations as "jubil-trumpet" and purposely changed "Joyful Hounds" to "Jubil Hounds." This would be likely, for example, if there was a clergyman anywhere in the chain of transmission.

3) Mr Pole (or his ultimate source), as Malcolm suggested, had "Jubilee" in mind.

Indeed, looking at the OED one is surprised by the extent to which "jubil" as a particle (not usually a whole word) is present in English. Words using it include: Jubel (a joyful cry--the OED specifically mentions a jubel as a cry "to dogs or the like"); jubilance, jubilant, jubilarian, jubilary, jubilate, jubilatory, jubilean, jubilist, jubilize, jubilose.

Since most versions give "joyful hounds," it would be a great coincidence if Mr Pole reached "jubal/jubil" by completely random chance. It seems much more likely to me that the common particle jubil- (which means joy) was the basis for the change.

One thing I like about any of these solutions is that Mr Pole is revealed as a man with either arcane knowledge or great verbal creativity, while RVW is revealed as a poor speller, rendering the perfectly good word "jubil" as "jubal!"

Just my twa cents!