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).