It depends on which origin one accepts. One is that it originated from the Gullah people (African-origin slaves on an island off S.Carolina) and the other is that it was taught by a missionary to Angolans, from whence it made its may back to the West.
If the former, then koom-buy-ah. If the latter, then come-by-ya ('come by here', asking the Lord to come and help)
But Gullah is a Creole, and has English as its origin. So one wonders just how the first syllable changed from come to koom.
My (admittedly limited!) experience of a few W African languages has been that there is no sound equivalent to the vowel in 'come'. But many, many words have 'oom' in them. For instance, 'tumbu' a nasty insect that lays its egg under your skin and hatches out from a hole like Alien. It's pronounced 'toom-boo'
I would therefore pronounce this as Koom-by-yah.