***** NOT A TEACHER *****
Great question, XXWZS:
The music is loud; the music in room 6 is louder than the music in room 7; the music in room 8 is the loudest of all three rooms.
As you can see, "loud" is an adjective in those sentences.
She speaks loudly; She speaks more loudly than her husband does; in fact, she speaks the most loudly of all the five people in her family.
As you can see, "loudly" is an adverb.
Here is the problem: Many native speakers do NOT want to take the time and effort to say "She speaks more loudly than her husband does." So they simply use the adjectival form: "She speaks louder than her husband." Since so many native speakers do it, can we say that it is "wrong"? I will keep my opinion to myself.
Interested in Language