The Akka IO API should provide a means of using the functionality exposed by the java.nio.DatagramChannel.join operations in Java 7:

http://docs.oracle.com/javase/7/docs/api/java/nio/channels/MulticastChannel.html#join(java.net.InetAddress, java.net.NetworkInterface)

Without access to this functionality, it is not possible to receive UDP datagrams addressed to multicast addresses.