Diagnosis
The default sound mode is flattening the dynamic range. You need to activate the dedicated dialogue enhancer and adjust the center channel.
How to Fix It
Press the MOVIE button on the remote to cycle to the 'Dialogue Enhancer' mode—the display will show 'DIALOG'. Then, press the CENTER LEVEL button (or VOL +/- while 'CENTER' is displayed) to increase the level to +3 or +4. This directly boosts the frequencies where speech lives.
If That Didn't Work
Check your source settings. On your TV or streaming device, ensure audio output is set to a multichannel format like Dolby Digital, not stereo PCM. Stereo signals won't utilize the center channel enhancement as effectively.
Heads Up
The 'Night' mode (press MOVIE repeatedly to find it) compresses dynamic range, making quiet sounds louder and loud sounds quieter, which can also help. Try it, but know it reduces overall impact for action scenes.