No can do. The H.323 protocol is not secure, and never can be. Ya gotta bitch at Intel for that one...
Passwords will not help. Doesn't matter much, tho' (see next paragraph)
However, it's HIGHLY unlikely someone will anticipate you will do "questionable" things with NM, and sniffing the stream is nearly impossible, since H.323 is point-to-point A/V only. The likelyhood someone will intercept your "transmission" is less than .001 of 1%. Maybe a good hacker could do it, but they have to know you're GOING to do it ahead of time.
Go have fun, you crazy kidz...