I think the only way to edit a voice post once posted is to do it completely over. I know I had a similar problem quite a long while back (I'd somehow skipped a whole paragraph and did not notice until I had posted).

I made a new voice-post that was private and copied the thingie and pasted it into the old post. I think it worked then, but that was a few years back and LJ's changed some things so I don't know if it would still work.

Actually, if it's just the forgetting to say the chapter, you could let it pass. It would not be the first time someone's done that over the years.