Sometimes it's not even obvious to a native speaker! But to me: - MFF? Is just a sound, similar to eh? or uh? - ULP! Is a swallowing sound, like 'gulp'. - TCH! imagine that. Tch is like 'tut'. It's a clicking sound you make with your tongue. - we're gonna catch the dragon "of cuzz" Of course, maybe? - Swell! I think that this is an old American expression, meaning 'great'. - T'heck What the heck?

Just to add to Gary's excellent explanation of these 'non-verbals' : 'Mff' tends to be used for the muffled noise you make when you need to make a sound but you can't breathe or open your mouth. Perhaps the speaker's mouth is full of cookies, or maybe they have just fallen face down in the mud! I'd be interested to learn what the situation was where you saw 'mff' used.