That's a new one. If you can't scrape it off you will have to find a to heat it so the chocolate will melt. I would find a repair shop and see if they might have any luck. If under warranty you could take it to Apple and hope they don't notice the chocolate.
If you can find a way to safely heat the chocolate/speaker you could press cheese cloth against the heated area and it should draw out some of the chocolate. But as they say on TV. Don't try this at home leave it to the professionals.
Good luck.