app-emulation/faudio should be utilizing MULTILIB_COMPAT in all of its versions since it is -* for most arches. Setting MULTILIB_COMPAT=( abi_x86_32 abi_x86_64 ) would align best with this package and not add new metadata as new ABIs are declared. See multilib-build.eclass for more details on what this does
Seems to be fixed in tree