There is no support for this out of the box right now. The voice packets are handled internally and either routed to the platform APIs (LIVE/Steam) or to our AudioComponent system (NULL) for playback.
You could modify code in the following place to intercept the voice packets.
UVoiceChannel::ReceivedBunch(FInBunch& Bunch)
This serializes the network voice packet and adds it to the VoiceData.RemotePackets variable on most platforms. If you take the packet yourself you’d have to put it back on that array or make sure to play the data somehow.