What is the recommended reaction to answers to questions that have been closed?
It depends on the closing reason and on the quality of the answer. Usually, the answers stay, which is ok in most cases. No need to penalize the answerers by deleting their contribution. It's still possible that it might help somebody, and I think most times the answers were given in good faith (perhaps sometimes additionally motivated by the reputation system, but that's life here).
In a case of a duplicate, answers may be merged, i.e. moved to the other question. That's a great thing if it matches, however often answers consides the specific case and not the possibly canonical question, so it doesn't fit. It would be great, if people would firstly check for a duplicate before answering. I often see answers to basic questions, which are dupes, even by high rep users. I believe flagging a duplicate should be rewarded more than getting rep for a fast answer possibly without checking.
But back to the topic.
Considering voting: I suggest vote as usual. If the answer is good, upvote. Is it offtopic such as the question, which was closed as offtopic, don't vote. Local view: answers to closed questions can still be sorted by votes. Global view: even if there's no reason to sort answers, such as in the case of a single answer of a closed question, you still could consider to give rep to the poster. But this should be for on-topic quality, i.e. not for off-topic questions, and not for the xth basic duplicate, do not encourage strict rep hunting. If it's a great post, vote up though (in that case merging comes up again).