Sorry, couldn't edit my previous post.
The ambient sound won't play at all, no matter if I set it to Off, Xbox, Costum or both. As you can already see in my sig, my xbox is softmodded. I used SID 1.01, so the BIOS i'm loading from my HDD is (IIRC) EvoX M8+. Perhaps it has to do with that, alltough the 543 build worked flawlesly.