XMMS is missing IPv6 support which is available for Debian version of XMMS.
xmms-1.2.7-r17 has been added to portage.
This ebuild has support for ipv6 added. Please mark this bug
RESOLVED VERIFIED if ipv6 works for you in this version and please
make a comment and mark it REOPENED if you experience further issues.
xmms-1.2.7-r17 does have IPv6 support. Works fine for atleast mp3 and vorbis
streaming over IPv6.