How do I loosen my flute?

It sounds like you won a flute that is too advanced for you to know how to care for properly. Yes, they can separate these joints - I would surmise that careless embly by you has caused the tenon to get out of round, and now it is binding - or it is really dirty, or you put something ON that tenon, thinking it was going to be a lubricant, and now it has seized. It is possible, if you are EXTREMELY careful to lay the flute on a flat surface and drip a DROP of WD40 into the joint, or oil (NOT regular oil!!!) and see if after a few minutes or hours, it loosens. They clean the inside and the tenon with a clean cloth until there is NO residue - and carefully see if you can re-emble it. Unless you have a LOT of confidence in your skill, I would not try this. I also have a Mura - and have never had any problems, and this flute is in daily professional use. Good luck - I hope you get this sorted. And in the future - get it into a GOOD shop, even if you have to send it to Jeff Weissman in NYC, or the Mura importers in Royal Oak, MI - I would bet that this flue has OTHER issues, too.