I have been racking my head now for the last few days over this.
We have had 3 xbox's setup on LAN. We have had no issues with in game chat on any of COD or HALO games or any others. We have just had the basic free membership of LIVE. The kids and I received Halo 4 for Christmas and we had no problem with the voice/chat in the game. We then went and used the code that allowed us to play online for 14 days and that was all perfect. I had bought the kids both a 3 month XBOX live gold cards. One of my sons in puted his and was playing. We decided to go ahead and play a system link game and we had the game lobby chat working but as soon as the game started there was no chat. We then tried CODMW3 and had the same problem. You can see the little speaker icons on the screen showing there is talk going on but no sound was coming out of the headsets. When he is on xbox live with his freinds everything works perfect. Now this is an issue with all 3 xbox's. I have gone and verified that all the voice settings are correct. I have reset the router. When i do the network system check test it says it fails with the NAT. Sometimes it would work though. The router is a FIOS MI424-WR. I assigned the xbox's static ip's. I am becoming frustrated because everything was perfect before any of the xbox live codes were used. As of now my two kids have there xbox live gold memberships and i still have just the basic free membership. Any help would be greatly appreciated. I am going crazy over this since it worked perfect before and there have been no changes to the router.