Well, as a former DD-WRT user I’d advise stronlgy against using it.
If you can, switch to HyperWRT thibor or plain openWRT. DD-WRT is way too unstable. At least on my Linksys WRT54s it was.
Anyway, also with dd-wrt there were no multicasting issues with me either. mt-daapd and the soundbridge worked just nice. 🙂