If you are using it remotely, try the tunnel option, see if that helps. One way audio is almost always a blocked or miss-directed port problem. If you, at the remote end, are not hearing audio, then that would point to an issue with the router/firewall at that end. Try bypassing the router, get a public IP on the PC running the 3Cx softphone, see if you get audio. Have you tried from more than one remote location (router)?