I for one loved Mighty Joe Young and especially "Windsong" which the webmaster also did not like. What can i say... he taste is lacking.(no offense of course)
The "singing half a note off" adds more urgency. The reviewer made a similar complaint in his Mighty Joe Young review. The technique distinguishes itself as native and human i think. It is authentic to that set of African music as i have read else where. Not all choirs have to be angelic and pitch-perfectly church like. I guess he(the reviewer) has a partial-cultural appreciation deficit.
Like "Windsong", "Amandla" (also reprised in the End credits) is a very moving piece. The soloist may appear irritating at first, but after several listens it adds an extra level of humanism and emotionality. Horner used a similar "African soloist over backing choir" method in Mighty Joe Young's Windsong (the second reprise of that theme) and in Beyond Border's Chechnya IV (the final song in the second reprise of the world/love theme) to excellent effect.
At least check out the main two tracks listed above - especially with the tone of the movie in mind (apartheid in South Africa). It's quite awe-inspiring.
and by the way, i would have given this score only a two or three star (and 4 for the main theme), but Mighty Joe Young and Beyond Borders are both some of Horner's finest works and are very underrated on this site. They are both without a doubt five star works.