Sorry if this is a bad answer since I'm new here, I personally think that it's how much emphasis you put onto it, if it's like shouting like in AAAAAAAAAAAlllen! it would be like that, but if it's something like Ch-ch-ch-ch-ch , it would have lowercase letters. I don't really have any references though, just my opinion.
Here's the difference between one capital and multiple capitals.
Ahhhhhhhhhhhhh!
AHHHHHHHHHHHHHHH!
Doesn't it give a effect? Also, doesn't it seem like the one capital one seem like a person sighing because they understand something, and the multiple capital one sounds like someone screaming.
So, in my opinion, my answer is that it varies on what you want the reader to understand. I always thought that it was such a simple question. Maybe you think something else though, but however it is your choice on what YOU want the reader to believe. Just don't use all capital letters in a whole sentence, as it really gives a headache.
References:
1. capital letters for emphasis, BtB
2.All caps hard for users to read *note: first paragraph