Diagnosis
The default audio mode is not optimizing for speech. You need to enable the dedicated Voice Adjust technology.
How to Fix It
Press the 'Voice Adjust' button on the remote. You will see 'Voice Adjust On' on the soundbar display. While content with dialogue is playing, press the 'Volume Up' button on the remote to increase just the voice levels. You can adjust from +1 to +10.
If That Didn't Work
If voices are still muffled, check your source device's audio output settings. Ensure it is set to output Dolby Digital or a compatible surround format, not basic stereo PCM. The soundbar's 'Movie' or 'Night' mode may also help.
Heads Up
Increasing Voice Adjust too high (+8 to +10) can cause distortion and make background sounds tinny. Find a balance, usually between +3 and +6.