Ok, the linksys WRE54 normally should already forward all thats neccesary. Can you move the soundbridge closer to the repeater just to check wether its a wireless reception issue?
The multicast settings are under Security – Firewall in the webinterface, usually.
Also, the WEG54 is essentially just a Linksys WRT54g minus the 4port switch, so you might as well try and set the WRE54 to wireless-LAN bridge mode and wire the soundbridge to it.
If all that fails you might consider flashing it with a replacement firmware.
I have found the linksysinfo.org forums most helpful. Replacement firmware even improves stability and features, whilst remaining easy to configure (same webinterface). I f.i. am running HyperWRT thibor on 2 WRT54G without a hitch for more than a year now.