LG 2019 TVs launched with a limitation in HDMI eARC that prevented PCM multi-channel audio from working. A firmware fix is now finally rolling out to B9, C9, SM90 and other models.
LG's eARC PCM fix
There are two ways to get multi-channel audio pass-through via a TV's HDMI eARC port; bitstream or PCM (uncompressed audio). The latter was not working at launch on LG 2019 TVs meaning that PCM multi-channel audio from consoles, PC and some other devices would not work.
It took more than a year but a firmware fix is now finally rolling out in the US and Europe to B9, C9, and other eARC-compatible 2019 LG models. After installing the firmware, you can get multi-channel PCM (5.1, 7.1, Atmos) pass-through via the TV's eARC port (you may need to change your TV's audio settings).
FlatpanelsHD has been getting several reports of users in Europe who have received the update via the built-in update function in their TV. Firmware updates roll out in stages so if you are not seeing it yet, check again later or consider updating manually with the download links in the table at the bottom.
Also read: HDMI ARC and eARC explained
The case has also helped but the spotlight on limitations in the HDMI eARC system. While the PCM limitation was an error in LG TVs, it would seem that TVs - not just LG TVs - do not necessarily to let DTS:X pass through eARC. Specifically for LG, the company has dropped decoding support for DTS entirely in its newest TVs. In other words, do your homework before relying blindly on eARC in TVs.