these are highly intelligent creatures and accomplished sorcerers.

Eh?

They're not completely animalistic (in Daggerfall they wandered around in heavy armor wielding an axe)...

But I've always thought of their poison/lightning/fire breath attacks as an innate attack, not a spellcasting sort of thing.

Is there a book I haven't read that says they're sorcerers?

Admittedly, there's not a lot of support. I'm basing this on two named Daedroth from Morrowind: Hrelvesuu and Menta-Na.

Menta-Na in particular seems like it should be intelligent. One of Molag Bal's chief lieutenants who has decided to stop spreading misery in Morrowind and hs set itself up in a snug little gave with rugs and candles and other such creature comforts. Admittedly it's a little flimsy.

On the other hand, we do know that scamps can talk. Even disregarding Creeper as being really Barbus, we have a talking scamp offering refreshments in Ald Daedroth and the ninny in Huleen's Hut describes talking to the scamp he summons. So I guess just because they don't talk as a rule, that doesn't mean they can't. We see the same thing with Hagravens in Skyrim.

So I think, on the whole, I'm on fairly solid ground. Might need to do some work in-game to explain it though

Very true, all of it.

In fact, how do we know they aren't talking? Intermittently or constantly?

If their capacity for vocal speech isn't in doubt, then perhaps it's their willingness.

Perhaps they only speak among themselves... they might consider speech with a mortal to be demeaning, only to be exercised in the direst straits, or when directly ordered. We may assume that the only races to communicate are the Dremora, Saints, and Seducers, and thus assume that a humanoid shape is a general prerequisite for speech, but this may only be a sort of species-centric conceit on our part. This quote:

 

The Dremora refer to themselves as 'The Kyn' ('the People'), contrasting themselves to other Daedra, whom they consider unthinking animals. 

(from Varieties of Daedra)

Could well be a cultural conceit, not necessarily a biological limitation. It could be that the humanoid daedra are among those who consider mortals as semi-equals - of a lower order, but still high enough to be worthy of speech. The Dremora, in particular, may venerate mortals as being (nearly) as capable as themselves in the arts of magic and war. The Mazken have always seemed to be more sympathetic to mortals, while the Aureals seem to take an Altmer-like stance toward "lesser races" - they are to be used as necessary, discarded when unneeded, and taught proper manners when appropriate.

Or, perhaps they (going back to Daedra as a whole) normally speak telepathically and thus have no reason to use an auditory language - the squeaks, grunts, squeals, shrieks, and growls normally emitted could well be the simple exhalation of the lungs past the voice reeds, much as you or I would grunt or gasp if we were struck.

Or perhaps they speak all the time, it's just in a language we don't understand. Aside from varying scripts such as Falmer, Dwemer, Daedric, etc. (that amount to nothing but replacement ciphers for a 26-letter alphabet, actually), there's nothing to say they don't have their own languages. Giants in Skyrim, for example... I'm of the opinion that they do talk, just their own language that sounds similar to vaguely word-like grunts. They're not grunts at all, but an actual language that we cannot understand (or possibly, hear - their words are low-pitched enough that subsonics could be involved).

The lore is complex and manifold, but it is also full of holes... some deliberate, some merely the result of oversight or accidental self-contradiction. Really, you could come up with any sort of justification for any sort of talking animal you want.

Could well be a cultural conceit, not necessarily a biological limitation. It could be that the humanoid daedra are among those who consider mortals as semi-equals - of a lower order, but still high enough to be worthy of speech.

 

For the Daedroth at least, I can imagine that maybe violence, degradation and subjugation are the most beautiful ways to resolve a dispute. That they see a pleasing elegance to rape and pillage that isn't found in any other activity. So they can talk, and they will talk when needed, but to them it's an ugly tawdry thing to do, that makes less of them and their victim both.

You know, I almost think I believe it...

[edit]

Tthere's a lore book to be had in there, somewhere...