That means that you have to re-download and re-unpack Kelsey mod with full audio files from Pocket Plane website to your web directory, and then carefully install it again according to the readme. Make sure to uninstall the previous version.
(I usually just delete mods, override folder, restore a clean backup and unpack and install mods again one by one, as per this thread:
http://forums.pocketplane.net/index.php/topic,17555 . But that might be a bit drastic for an unexperienced user).