I just tried it. If talkback is sent to a Flex Mix and then that Flex Mix is sent to an EarMix, the Talk button works. If you try to directly route the Talkback mic to an EarMix via AVB, the Talk button does not work. I assume this is how its meant to be because the Talkback Edit window only allows you to select Flex mixes or the Main to send it to. This could be a good feature request though.