Explanation

"It is delightful to hear her speak as she speaks melodiously ." 

Explanation:

Melodiously means "musically or pleasantly," which fits the idea of speech that brings delight.

Slowly means "at a slow pace," which doesn't necessarily imply delight.

Rhythmically means "in a regular, repeated pattern," but it doesn’t always mean pleasant.

Softly means "in a low voice," which doesn't convey musicality or delight.