Well there are no 'm' sounds (he doesn't close his mouth enough) but a definite 'aa-aa'.
I am ashamed to say he has been thoroughly coached by dh, with lots of 'say mama' (the dds call me mama as well) - then this morning when he came to tell me breakfast was ready, dh stood behind him and said, 'say mama' and he said 'aa aa'.
His only noise is usually a back of the throat 'uh uh uh' sound (well apart from screaming - which he has been practicing for the last hour) so this sounds really distinct.
I wish I could work out how to get the video to work, but I guess you can all imagine how huge it feels without seeing it!