I'm not aware of Cisco simply replacing the batteries for CP-8831-MIC-WRLS=, but it's possible they do. I suspect they would RMA the whole mic kit, however, the best way to know for sure is to open up an RMA if your company has a Cisco service contract.
If your company does not have a service contract, there is still a warranty for the unit. Contact support and advise of the situation and verify the warranty.
Apologies...I don't know of another way to acquire replacement batteries.