I don't know of any evidence that large numbers of slaves were
Muslim. Given what we know about their origins in Africa, that
assertion seems most unlikely. However, there are individual
narratives of literate, cosmopolitan slaves who were Muslims and who
even succeeded in practicing their religion in the New World (mostly
in Jamaica rather than on the mainland).
I don't have the reference at home, but Ivor Wilks is one of the
editors of a book that contains the narratives along with informative
introductory essays. I'll send the exact reference next week in
response to a request.