"DURSLEY!" he boomed. Uncle Vernon, who had gone very pale, whispered something that sounded like "Mimblewimble." Hagrid stared wildly at Harry.
"But yeh must know about yer mom and dad," he said. "I mean, they're famous. You're famous." "What? My -- my mom and dad weren't famous, were they?"
(Harry Potter and the Sorcerer's Stone)
What’s the reason that “Mimblewimble” starts with a capital letter?
In this case, it seems that capitalization was a rather arbitrary decision.
It could be argued that it is capitalized because it is the beginning of a "sentence", though really it is just a single word, and a made up, gibberish word at that. In this case, there really is no concrete rule about whether to capitalize or not. Since it's a made up word, it could be a proper or improper noun, a proper, full sentence quote (requiring a capital) or simply an excerpt (not requiring one), both at the discretion of the author.
A sentence, even a quoted sentence, should begin with a capital letter. This is true even if the sentence is short and/or not grammatically correct.
For example:
He whispered something that sounded like, "It is sunny out."
We are quoting a sentence, so it should be capitalized.
He whispered something that sounded like, "Sunny out."
It's not a complete sentence, but we still capitalize.
We normally capitalize exclamations by the same reasoning:
He shouted something that sounded like, "Hooray!"
But if we are quoting an excerpt from a single sentence, then we don't normally capitalize.
He insisted we leave "in the morning".
This case is ambiguous because it's a nonsense word, so we don't know if it's supposed to be a complete sentence, an exclamation, or just a fragment. I would have capitalized it also, as it seems like the implication is that it's quoting the entirely of what he said, which would presumably be construed as a full sentence. But it's debatable.
Quotations should start with capital letters since those are sentences themselves.
For example if we say the following :: This has definitely been my best birthday yet.
We start with a capital letter using the normal sentence grammar rules. Now when we put this into a quote. It becomes::
He whispered to his mother, "This has definitely been my best birthday yet."
It stays the same. All sentences must start with a capital letter. The only reason to not start with a capital is if you are talking about something whose proper/common name starts without a capital letter.
For example ::
iPhone usage in Canada has now surpassed Android in statistics. This would remain the same in a quote, example : He said with authority, "iPhone usage in Canada…"
In the case above, since the person is mumbling, we're not sure what he said. Therefore we assume a capitalization of the sentence. If we knew that he was talking about say iMax 3D, there could have been a non-capitalized sentence.
