I've been reading about Iyanden lately but a question hit me I wonder if the dead can talk to the Eldar outloud or through their mind. Does anyone know?

Depends upon the author. It's been contradicted many times over in multiple sources, with even the bile brimming canon-defiling garbage called Codex: Iyanden simultaneously saying they can and can't many times over. The most numerous depiction tends to show that Wraithlords at least retain a good deal of sentience and self control, and tend to have speaking roles among their kind. Wraithguard tend to speak to some capacity, but it's more often shown via Seers or through telepathic means rather than any vocal method.

Well the Eldar Retribution story seemed to indicate the information that Taldeer uncovered regarding Necron Tomb Worlds was useful to the Craftworlds even when she was very dead.

so they basically work exactly like dread and seer are the techmarine equivalent.

In at least some canon sources they can physically talk. I believe the rote explanation is that wraithbone vibrates to produce the sound, since they obviously lack the organs associated with speech.

She may have joined the infinity circuit, as all Eldar souls do if recovered. From there she could communicate with a Seer, afaik.

In a Dreadnought there is a body encased in machinery, even if said body is wounded and broken. In a Wraithlord, Wraithknight or Wraithguard the only sentient element is the soul of an Eldar.

the result is basically the same: they are alive but not totally, the mind is barely connected to the real world and need help/support to stay in this world and interact wih it. both are extemely revered respected and put in rest when no needed awoken only when there is a real need. etc etc...

Talk? No. Communicate? Yes. How would the "Ghost" houses of Iyanden organise themselves?

TL;DR: Maybe.

So far as I know, they cannot talk out loud, but they can communicate mentally. Spiritseers can communicate mentally with Wraithguard (and subsequent Wraith-types, hereafter Wraithetc) as they help with their transfer from the Infinity Circuit to the physical wraithbone shell.

A unit of Wraithguard used to need a Warlock to direct it in the tabletop game or it would tend to become disorientated, and the impression I got was that this was due to them perceiving the world around them with psychic senses. The more developed the psychic mind, the more it 'shone' and the easier it was for the Wraithetc to see. I understood them as communicating in terms of broad brush feelings and emotions rather than words when in this state. I'm not one hundred percent sure this is not my interpretation of a lore void, though, to be fair I haven't played the game in a long time, but I would guess that this role is now for the Spiritseer. Not that I'm against more (sensible!) development of the Eldar psychic Paths I interpreted this to mean that a Seer-trained Eldar could communicate with a Wraithetc, but an untrained one might struggle.

Ages ago, back when Morai-Heg was young there was a Path called Infinity Matrix Technician, who worked with the Infinity Matrices and the devices that they directed. I don't think that this Path has gone, exactly, but I haven't seen it mentioned in a long while. Such a person would also need to communicate with the dead, but this time within the Circuit. Although the world within the IM is noted as being composed at least as much of thoughts and feelings as sensory perceptions, I don't think an Eldar loses their individuality in there. They just become used to being without body.

Some sources give the Eldar the ability to communicate with each other mentally via telepathy, which might need runes or might not depending on who and the source. I am truly sorry for the jumbled mess of that sentence. *sigh* Such is the ambiguity of ... just about everything Eldar. But the possibility of communicating mentally in words exists in the sources, so it wouldn't be completely out of left field for you to say that Wraithect could do it.

If you think about your own thoughts, how many of them could definitely be pinned down as individual words and sentences? That's what I think we are dealing with here Everything from realising that you are hungry and eating, to composing and reading over an email are thought processes and if someone could tap into them, I don't think many would be words. On the other hand, if I use the word telepathy, I know that I mean narrow-band, word-only, consciously directed communication. Can Wraithect do this? ... I don't think either GW or myself are sure I'd say yes with those psychically trained to do so, e.g. Spiritseers, but not Joethriel Guardian. For them it would be impressions and feelings at most.

Does anyone know if they ever fleshed out what it is that differentiates the alert, speaking sentience of a Wraithlord from the rest?

I'd like to propose that Wraithlords are the souls not just of Eldar warriors - but Witches in particular. They would possess a much stronger psychic link to the material world - able to perceive and communicate even in death. Perhaps they are able to project their voice aloud, psychically through something like a voice synthesizer within the wraithbone.

If they psycho-kinetically generate soft breezes through a wraithbone 'throat' they might mimic a voice: albeit hauntingly (as they do). This would explain why other Eldar Warriors may in death become Wraithguard, but are unable to communicate verbally as the Lords do - they lack the psychic precision to operate an instrument-like Wraithbone Throat with their mind, while also in the heat of battle, while also controlling a robot.

So if lords are Witches (Locks / Seers, etc), that would explain both their better alertness while possessing a Wraith-walker, and it would explain them having a voice where others do not. It may even explain their preference to melee weapons - given that Locks and Seers usually wield Witchblades in life (which Wraithlord blades look a lot like) or Singing Spears.

Even their shoulder-mounted ranged weapons would make more sense (compared to the held weapons of the Wraithguard) - a Witch is used to attacking at range with a psychic projection of their will - it may feel more natural to them for their attacks to be sourced at their shoulder/head height rather than a weapon wielded in their hands like a Shuriken Catapult.

Edit: Oh man, I really hope we get a Wraithlord Hero class at some point - can you imagine struting up through the map - stomping on an SM who thought to take cover against a puny wall - almost skipping up to a speeding Rhino and dicing it in half - spewing a tide of shurikens to suppress an entrenched enemy firing position that had been preventing your teams advance across an open field. Wraithlord Hero... yessss...... gooood... neeedddd!