I don't think it is possible. This Q has been asked in one form or another in the past, and I think the answer was that there is no way to "get out" of the mission.pbo when configuring sounds in your description.ext. So basically, yes, you have to include the sound with your mission.
The only exception, of course, are sounds that are already configured (in the game's or an addon's config) to be used in game. But for sounds that aren't configured that way, I think you are SOL.
Hopefully someone will correct me and say I'm wrong, but I'm pretty sure that was the answer.