Diagnosis
HDMI 2 on this model is the dedicated high-bandwidth port for 4K@120Hz. The audio handshake can fail if the TV's 'Audio' settings aren't configured to accept the signal your Xbox is sending.
How to Fix It
On your TV remote, press Menu and go to Audio. Set 'Digital Audio Out' to 'PCM'. This is the most compatible setting. Next, go to 'Input Settings' for HDMI 2 and make sure 'Full UHD Color' is turned 'On'. Finally, on your Xbox, go to Audio settings and try changing the HDMI audio format from 'Bitstream' to 'Stereo uncompressed' to test.
If That Didn't Work
Try a different, high-speed HDMI cable. The one that came with your Xbox might not be up to spec for the full bandwidth this port demands. Also, try plugging the Xbox into HDMI 1 or 3. If you get sound there, the issue is specific to the HDMI 2 port's configuration.
Heads Up
If you're using a soundbar or receiver via the TV's ARC/eARC port (HDMI 1), there can be a conflict. Try disconnecting the soundbar, getting sound working directly from the TV speakers with the Xbox, then reconnecting the soundbar.