You can still use them that way, although the preferred method of distributing campaigns with their own voice overs/sound files is by packaging them up in a .vp. For the "old fashioned" method you speak of, simply put the speech files you want in the data/voice/X directory of youre choosing. Replace "X" with "briefing," "debriefing," "special," or "command_briefings." data/voice/special should contain all the voice files you want played in-mission (while you're flying around, not the de/briefings).
Sound effects such as explosions and weapons fire are another matter, and depending on whether or not you're overriding existing sound effects or making new ones it can entail some editing.