How to Stop Talking in Your Sleep
You can look into sleep apnea mouth guards that will muffle what you say, but there's still a chance your words may be intelligible enough to understand or worse yet, misinterpreted. You could be wearing a mouth guard while sleeping and say "nuggets" and someone on the plane might hear something completely different leading to a big brawl and worse yet, an interrupted night of sleep.
The best way I've found to deal with this problem is to simply place duct tape over your own mouth before you plan to go to sleep. This will feel awkward at first unless you've had a particularly horrifying childhood, but you get used to it pretty fast. It will look a little strange to others, but the great thing about being asleep is you don't have to bother with people's ignorance anymore. Leave them behind and enter the amazing world of dreams! You may have a lot of dreams of being kidnapped at first, but this will usually fade away in the coming months. If it doesn't I would recommend not using duct tape anymore and seeing a therapist because there might be some past issues you have unresolved in your subconscious which may be the reason you're talking in your sleep to begin with.
Warning: Do not cover your mouth with duct tape before sleep if you have just been out drinking a lot, doing drugs, or just ate weird ethnic cuisine or seafood as you may vomit in your sleep and the duct tape will force the vomit down your trachea, perhaps killing you.