'sound ' can be used as an adjective as well as an adverb. So I can say 'sound asleep'. 'soundly' is an adverb. So I can say 'soundly asleep'. My question is: is there any difference between 'sound asleep' and 'soundly asleep'?
Looking forward to hearing from you.
Thank you in advance.
He was soundly asleep with the storm struck.
It is an adverb and can be used basically anywhere that "sound" can when it is used as an adverb. You might not use "sound" as an adverb here although from a grammatical point of view there is nothing wrong.
He crept into the warm bed and slept soundly through the night.
There's nothing wrong with "He was soundly asleep" except that it's unlikely anyone would say or write it.
"He was sound asleep" is normal.