On my mostly stable amd64 wine-1.7.3 fails to build with this message:
[ebuild U ~] app-emulation/emul-linux-x86-baselibs-20131008-r1  USE="development" ABI_X86="-32%" 54,680 kB
[ebuild U ~] app-emulation/wine-1.7.3 [1.7.0] USE="X..." 65,938 kB
Total: 2 packages (2 upgrades), Size of downloads: 120,618 kB
!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:
(app-emulation/emul-linux-x86-baselibs-20130224::gentoo, installed) pulled in by
~app-emulation/emul-linux-x86-baselibs-20130224 required by (app-emulation/emul-linux-x86-soundlibs-20130224::gentoo, installed)
(and 4 more with the same problem)
(app-emulation/emul-linux-x86-baselibs-20131008-r1::gentoo, ebuild scheduled for merge) pulled in by
>=app-emulation/emul-linux-x86-baselibs-20131008[development] required by (app-emulation/wine-1.7.3::gentoo, ebuild scheduled for merg
Mixing is not really supported.
On top of that, ABI_X86 even in unstable is still a bit quirky.
Try setting ABI_X68=32 on app-emulation/emul-linux-x86-baselibs.
But this doesn't seem to be a bug, but a support question - use forums.
This happens when mixing stable and unstable packages.
You'll want to either allow (accept keyword) a newer version of emul-linux-x86-soundlibs or you'll want to disallow (unaccept / mask) version 20131008-r1 of emul-linux-x86-baselibs.