According to the rules, you need to use the adverb to modify a verb: "He speaks loudLY." That is, he speaks in a loud manner / way. "Loud" is an adjective. It describes a noun. "The NOISE from the airplane is loud." It describes the noise. Sometimes native speakers use "loud" when the rules call for "loudly." For example, some native speakers say, "Don't speak so loud!" In other words, some native speakers are using "loud" as an adverb.

Loudly

Also, note the common phrase "I hear you loud and clear." In theory, it should be "... loudly and clearly," however people almost always use the first, "incorrect," form.