But if you accept that the voice of the player character impacts the intent, you also accept that the voiced reactions of the NPC's to the player character do so. Sure, you might say "my Warden was just being sarcastic and the NPC didn't pick up on it" but I can make similar excuses for my Hawke.
No, the voiced reaction of the NPCs only show
their intent. I can easily say that the NPC didn't pick up on the PC's tone; it's entirely believable, and often occurs in actual conversations.
I think that the case of a person attempting to say something in a particular way, but not at all managing to, is much less likely (if most people are angry, they'll sound angry; if they're upset, they'll sound upset, and so forth). I suppose you could decide that any given thing is being said that way for a sarcastic or mocking reason, but that's the only different way you could possibly interpret the voiced lines without trying to ignore the subtext entirely.
I'll admit I'm probably more sensitive to the whole subtext/intent issue due to doing so much acting and directing than many people are, but for me, hearing that shows one very definite interpretation and there is really no wiggle room (except possibly deciding that it's being said in that manner for a mocking effect). Hearing the NPC's subtext only shows the intent behind their words; it doesn't explicitly say anything about what your character did, while the voice acting does. There are, in fact, several different ways that your character could've said something that the NPCs could entirely reasonably react to in the same way, but that would say noticably different things about their personality.