I just finished a promo last night with not much AE work in it, but enough to justify rendering the entire 30 seconds. The original edit in FCP had wild sound, lip-sync sound, sound effects, music and voiceover announcer.
When I rendered my 30-second comp, I included NO audio. Just video. I brought it into a duplicated FCP edit timeline, turned off the existing video tracks, added the recently-rendered video......
.....and everything worked like a charm. Even the lip-sync stuff. No problems whatsoever.
The moral: you rarely, if ever, need to render audio in AE.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA