You're being taken for a ride.

Possibly the best thing to do is to accept the return and hope that they send them back in good condition.

I thought there was a time limit for buyers to open a case anyway - did they open it 30 days from date of order, or 30 days from date of delivery?

You could call the buyers bluff by accepting the return (and this is assuming that the buyer has actually opened a case for Not as Described"). Sometimes the buyer will not return the item as they hope you will just cave in to their demands.

Also, you could message the buyer that if you accept the return, and he does not send them back in good condition, you will pursue either a small claim in court, or involve a debt collector as the buyer would then have the speakers and the money and you can prove a loss to yourself.