You're not really supposed to say acronyms as words but spell them out.
Let's look at the second half first:
Since nothing follows the second half it is simply "MA" as in math or... I guess "mah" as in marble. We don't change the vowel to a long sound. Just don't say muh. It's or ma or mah (2 different sounds).
The first half could technically be pronounced "jay" however if you were to look at why, the reason would be that there's a vowel in the second half after a consonant. But this is a very soft rule in english. For example "rule" "knife" "bike" but then "decathalon" (latin formed) "liver" "water" "caress" et "cetera" (latin).
And so we would pronounce it in a way that would describe it to someone who's never seen it written before.
"ja-" (as in jack, or jam), or,
in the following sad yet acceptable pronounciation that many probably can't hear the difference in,
"jah" (as in Jah [religous]) causing some people on hearing this write "ja",
so that leaves us with 1: ja/ma 2: jah/mah (mix and match at your own leisure I suppose)
Canadian English-speaking, reading, and writing (Canadian English is extremely close to British English)