Why does NPC dialogue get so quiet when I look away?

That sounds like the audio may be reacting to direction or distance, though it could also be a bug. As a quick test, turn toward the speaker and stop moving for a moment. If the voice becomes clear again, that points to the way the sound is being positioned rather than the dialogue itself.

Checking the audio options for spatial or surround effects might be worth trying too. Change one setting at a time so you can tell whether anything actually helps.

For now, facing the character during important lines is the simplest workaround. If it keeps happening, note whether it occurs in other conversations too and send in a report with the scene and your audio setup. That gives someone a clearer way to reproduce it.