I believe you could do that via tha MLHSPlugin. You can experiment with the syntax following examples in the MLHSPlugin documentation. You would need a parameter in there of which client.
Or, you can use the new MainLobby Client ability to pass audio to a client. See the latest release notes for MLClient.
Or, you can use MLSpeak for speaking via the server. And very likely MLSpeak will also be broadened for client usage as well, but that doesn't exist today.