In the film Rogue One: A Star Wars Story, there is a scene at Darth Vader's castle on Mustafar where we can see Vader fully submerged in bacta fluid inside his rejuve tank. Then, after the announcing, from his attendant Vaneé, of Director Krennik's arrival, the tank began to empty and we can see part of Varder's head which does not appear to wear any hearing aid. I ask, is there a canon or legend explanation about how is possible that Vader can hear his attendant’s voice?
Fluid
We do not know about the Bacta fluid, though. So it is hard to be exact. Let's assume it is at least like water or maybe more viscous. As a rule of thumb you can say, the more viscous a fluid is the better it can transmit vibrations, aka sound.
Tank
The tank itself is probably made of glass. A hard material with a crystalline atomic structure. Also it is transparent, which is not only nice for light to pass through, it is also an indicator that it is able to transmit vibrations (sound) well.
Dampening
In both cases the sound may not be transmitted in exact form. Some degree of dampening or dispersion will occur. Also the sound maybe skewed or even certain frequencies are fully absorbed.
Given the small size of the tank, I would argue that those effects are not strong enough to dissipate the sound beyond recognition.
A beneficial effect also is, that the fluid seems to be perfectly in rest. No waves or other turmoil, no bubbles, that might disturb the sound paths.
Real World Examples
There are a few ways to give a little proof to the concept that glass and a fluid like water allow for sound transmitting into the tank:
Just put on your ear on the glass window and check for yourself how much of the sounds you are able to hear from the outside. In my experience this is plenty and far louder than you would expect. In most cases you will hear cars passing by though. That is of course just noise.
Ever had some noisy neighbors chatting in front of your (closed) window? It is not a problem to discern words and whole sentences even if being on the other side of the room (given there is not much else noise, as cars passing by).
There are laser microphones, which are able to use a window as measuring surface to be able to eavesdrop spoken words inside the room (The article mentions an infrared beam used by the Buran eavesdropping device in case of glass windows).
Fill a bathtub with water, put up a music player outside, and submerge yourself carefully (try not to make waves). It is possible to listen to the music and not even too bad.
You can check out this video of Veritasium, where someone is submerged in a similar tank and holds his breath for 17 minutes. The relevant parts are at 15:00, 16:45 and 17:00 minutes, where he asks for the elapsed time (by signing), and the answer is given verbally to him, which he then acknowledges (by signing).
So far no one has mentioned that Vader is a freakin’ cyborg. He could very well have the implants to hear stuff without needing earbuds.
In a technologically futuristic society, he could easily tap in to any audio pickup. So when Vaneé enters (let alone speaks) he could already be listening, else review what she said after the fact.
The article on the Telepathy force power on Wookieepedia says, "The telepathy of most Force-sensitive individuals was not potent enough to truly read the thoughts of other beings, and could only sense the emotions or general mindset of those around them." Darth Vader could. This article says:
"―Darth Vader uses telepathy to read Luke Skywalker's thoughts and learn of the existence of his daughter, Leia Organa3".
However, I would think force sense would be enough to sense that someone was present and talking, without knowing what they were saying.
If he didn't just know that they were there through his super strong force sense, it's always possible that he was already monitoring the thoughts of this person prior to the event. He might have considered an intrusion on his tank to be somewhat of a danger, due to his vulnerable, exposed, or immobilized condition, and force sense is well documented to be able to sense danger (and if he didn't, it's possible that the person was actually going to harm him, but his sensing danger thwarted this).
If he had considered it a danger and wasn't monitoring thoughts prior to the event, the reason he didn't react sooner would possibly be because he used his telepathy after sensing danger to find out what was going on. It's also possible he could tell the difference between a threat to his dignity and a threat to his life.
Probably the most realistic view, though, is that he was using force sight the whole time he was in the tank, and read the thoughts or lips of anyone he saw.
So even if his tank was fully sound-proof, it's still possible through the most basic of force powers (force sense), or more advanced ones (like reading someone's thoughts through telepathy).
