According to my observation, the traditional general rule is that we use "very much" or "much" instead of "very" before predicative adjectives beginning with a- such as alike, alive, afraid, ashamed, alone, awake, and asleep. However, nowadays it seems pretty common to say very alike, very afraid, very ashamed, but wide awake, and sound asleep. Language is quite arbitrary, isn't it? Please consult "Oxford Collocations Dictionary for students of English," Oxford University Press, 2009. Sincerely, Maoyueh
Sound asleep has been a common collocation for as long as I can remember, so it's not such a recent change, though I do agree that the very much is not as common as it was.