Sound and Soundly, quite confusing!

[kwfine]

According to the Cambridge dictionary, "Sound" can be used as an adverb.
Here is an example:

[Mister Micawber]

.
Sound asleep is the only collocation in which sound can be used as a an adverb; otherwise, use soundly.

Several adverbs have two acceptable forms: sound/soundly; slow/slowly; direct/directly; fair/fairly; loud/loudly; etc. Sometimes, as with sound/-ly they are used slightly differently.
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_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber