I wasn't even aware there were special holiday cards until now! I'll add them in the next update, just for completeness sake.
As for the sounds, how large are they? I've been attempting to trim back the plugin size and installation time to make it easier for people to get in to Lackey for Magic, since our community is rather small. Adding 168 files to the initial download would be rather counter intuitive from this standpoint, so I'm not willing to merge with your expansion.
Lackey used to (or it was planned to) have a feature to replace the sound column of the data file, depending on certain criteria like Type = Elf or what-have-you. Unfortunately this function doesn't work right now, and it might never work.
Alas, you don't need permission to make a plugin to Magic.
And as long as plugininfo.txt and the names column, sets column, number and order of cards in the card data files aren't modified, you won't get any out-of-sync problems either.