The soundbanks are just standard xbox wavebanks/soundbanks, you can even open them with my Monkey Island Explorer tool. So making a custom soundbank using the XACT tool should be completely possible, all someone needs to do is make a template with the correct names in the soundbank in the correct order then all anyone would have to do is add the sounds and compile it. Whether it'd work online is another matter though:
The game seems to do a crc check on its files, if any of them are changed then online play is disabled. Its completely understandable but also a shame because the interface is completely moddable - have a look in chat.txt, you can change the size of the hulking great chat box.
Custom soundbanks might work ingame though, perhaps they dont do a crc check on that folder.