Without the hyphen, it is obvious that the adverb "newly" modifies the adjective (participle) "opened," so there is no room for confusion or ambiguity. " On the other hand, people in the US are fairly liberal about using hyphens in such a case, so it would not probably be considered a mistake by most. As far as sound goes, I actually prefer the hyphen so that it sounds like "newly opened" goes together.
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