Ignore the original context.I agree that it would be better not to use 'as', for the same reason the passive is best avoided! It weakens the sentence. But "it doesn't sound as an assertive thing to do" is definitely not acceptable for several reasons.
The verb 'sound' is followed by an adjective such as good/wrong used adverbially because 'sound' is like 'look' and 'feel', verbs 'of the senses'. It can also be followed by 'like' + a noun - "That sounds like a good idea!".
'As' can't replace 'like' here.