'Speak more loudly' was accepted, to be fair to Duolingo, but their translation 'speak louder' is ungrammatical, louder being an adjective and it's the adverb, loudly, that is required, because you are qualifying a verb, not a noun. Speak louder is indeed commonly used, but it's a misuse.
Sorry. "Louder" can be either an adjective or an adverb.
This is rightly translated, "Speak more loudly," It is NOT "Speak louder." "Speak louder" is incorrect English. It is OK to use it but not to require it, and not to deprecate the correct answer. Not sure why "louder" is wrong here - I think it is an adjective and can't modify the verb. It is OK if it is part of a comparison "He speaks louder than she does." But here there is no idea of what the comparison is. Speak louder than what? is the deserved riposte.
"Speak louder" is an acceptable informal way of saying "Speak more loudly", but as you say, there is some controversy.
It is not necessary these days to conform to the strait-jacket prescriptivist rule that all adverbs must end in"-ly", but of course it can sound odd to some people - including me, sometimes!
I have the same question as i've noticed that DL is doing that on a number of different verbs. To summarize from the answer given below from mojavejeeper, I think you are saying that they do this when they use an exclamation mark. However, the sentence that I was questioning from DL is this: Mr. Lopez, speak louder, please. No exclamation . I think it should be habla so I'm confused as they used hable.
Exclamation marks aren't required to indicate a request (command form). In this case (since we're being formal, as indicated by the "señor"), if you were to put "habla" it would mean he speaks louder, rather than requesting him to do so.
"Señor, habla más alto" = Sir, he speaks louder
"Señor, hable más alto" = Sir, speak louder
Thanks, I like replies that give examples and don't use a lot of technical words for grammar. Though I am old enough to have taken proper grammar classes in grammar school, I really don't want to have to remember what I have forgotten. Here is a lingot for giving me a memoritic for this situation.