Laura, what you are describing is consistent with laryngitis. This is very similar to us when we yell in a sporting event or concert and we loose our voice for several days. In this case, your cats may have been meowing/hissing a little too much at the kitten. Usually, with rest of the vocal cords, it will restore the voice within 1-2 weeks. It is also important to remember that there could be an underlying medical disorder like laryngeal paralysis, tumors of the larynx, infections, etc. However, with a history of a new kitten and happening in two cats I am very inclined to believe this is related to hissing at the kitten.
I would recommend you do your best placing them in an area where they are not meowing or hissing for the next week or so. If no improvement within the next couple of weeks I would have them examined by the family veterinarian for a hands on examination. If you notice any other symptoms develop like a nasal discharge, anorexia, lethargy, etc. then have them checked immediately. Otherwise, I would separate them from the kitten to give their vocal cords time to rest.
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