Do you have a surround sound setup? If not, make sure dynamic range is set to headphones in the audio settings.
I have a 5.1 setup, never had this issue before. Sound is set to dynamic and all sound is working perfectly except on the focused character (and during all cutscenes). Changing the setting to headphones does nothing, including when headphones are plugged in.
Normally the issue with muffled voices is when the game outputs dialog through the central speaker which is either missing or not working.
Is it only the main character, or also other characters in cutscenes? Have you tested your speaker configuration in Windows to confirm the central speaker is working?
It could also just be an issue with the game, in which case you should try repairing it by right clicking DAI and clicking 'repair game'.
