I always thought it was Mufasa, as Silent_Cola said, mumbling it first as he tried to get a grip on the cliff, then repeating it louder for Scar to hear. I also thought that maybe since he did say "Scar" first, that maybe he realized that Scar himself wouldn't be flattered by Mufasa simply giving him a command, using his (nick?) name, and right afterward felt he should personalize it more, and refer to him as his brother?
"Scar! Help me!" = pushy; a king ordering one of his subjects.
"Brother! Help me!" = a family member pleading for help.
I always noticed that "brother" is said twice but I never gave it a second thought til now... because now I want to go rewatch it and see if that could be, as you (Silent_Cola) suggested, Scar saying it and not Mufasa? o.O Hmmm. Would be interesting. Similar to the above reason why Mufasa changed his mind and said brother in place of Scar, but instead Scar had to (bitterly) remind him they were brothers?
EDIT: I watched the YouTube video, I think the quality of the video makes it kind of difficult to tell, but from what I heard I'm unconvinced that that's Scar's voice... but now my interest is piqued; I'll keep my ears open next time I watch my Lion King DVD
Maybe it's Mufasa's concous of something, telling him to not order a subject around, but to plead to a family member for help...
