Diagnosis
Incorrect HDMI-CEC or audio format settings are preventing the handshake between the Cube, your TV, and the soundbar.
How to Fix It
Check the physical connection first. The Cube must connect to your TV's HDMI ARC/eARC port. Then, on the Cube, go to Settings > Display & Sounds > Audio. Set "HDMI Audio" to "Best Available" and ensure "Dolby Digital Plus" is ON. Most importantly, go to Settings > Equipment Control > Manage Equipment > TV and run the "Change TV Settings" setup again to re-establish CEC control.
If That Didn't Work
Bypass the Cube for audio. Connect the Cube to a regular HDMI port on the TV, and use a separate optical cable from your TV to the soundbar. Then set the Cube's audio output to Dolby Digital.
Heads Up
Some TVs require you to explicitly enable the ARC/eARC function and set the TV's digital audio output to "PCM" or "Passthrough" in the TV's own sound settings menu for this to work.