Let me butt in here.
sound is a linking verb. For this reason we say
It sounds natural, not
It sounds naturally.
Consequently, things are
natural sounding, not
naturally sounding.
sounding is a (present) participle. As such, it has some
qualities of a verb; it is not purely adjectival in character.
That's why the adjective
natural can combine with it. The adjective
natural 'modifies' the verbal aspects of the participle
sounding
as an adjective complement, even though the participle itself is
adjectival with respect to the rest of the sentence. This
analysis, while somewhat complicated, shows why this is not a case of
an adjective modifying another adjective, even though it appears that
way.
For the same reason, we say that people are
good looking, not
well looking.
CJ