All the current portage mbrola versions including the latest (stable) app-accessibility/mbrola-3.0.1h-r4 do not work on amd64-no-multilib system as the binary installed is 32-bit. Suggest adding the app-accessibility/mbrola package into /usr/portage/profiles/default-linux/amd64/200?.?/no-multilib/package.mask
Done