Using Teamviewer without Internet

I want to use Teamviewer to remote control and hear voice and see video on my LAN rather than over the internet. I can connect OK by using the IP Address which allows me to control the remote PC, but can't get voice to go through from one to the other, and video only goes from the remote to the local PC, not seeing the picture from the local PC on the remote (just a silhouette head in the little box at the bottom right of the screen). When I connect on the LAN using a modem connected to the internet it all works normally - ie voice goes both ways and both PCs can see the camera output from the other.

Are there any settings I need to apply to get it to all work using the IP address rather than the access ID which comes from the internet connection so that I can use it in a situation where there is no internet?

Thanks,

Janet