If you cannot get audio at resolutions over 1080 on your 4K TV, try the following steps:
1. Disconnect the HDMI cable from the TV.
2. In the the TV's Menu and look for a setting called "HDMI UHD Color" or similar.
3. Enable this setting, then disable it again.
4. Plug the HDMI cable back in.
You should be able to get audio over HDMI now.