Diagnosis
The TV's audio output level is being limited by an incorrect digital audio format setting, especially when using optical or HDMI ARC.
How to Fix It
Change the Digital Audio Out format setting.
1. Press Menu, go to Audio, then Digital Audio Out.
2. Change the setting from 'Auto' or 'Bitstream' to 'PCM'.
3. Exit the menu and test the volume. You should notice an immediate, significant increase in level.
If That Didn't Work
Check if 'Volume Leveling' (in the same Audio menu) is turned ON. If it is, turn it OFF. This feature normalizes audio and often makes everything seem quieter.
Heads Up
If you're using HDMI ARC, also ensure the CEC settings (System > CEC) are configured correctly, as a handshake issue can also mute or lower audio.