[H]e cleared his throat a little – taking pains, of course, to do so in a very muffled manner, since this noise, too, might sound different from human coughing, a thing he no longer trusted himself to decide. (1.23)
At this point in the story, Gregor starts to lose his self-consciousness about his human-animal split. In Quote #2, you'll recall, he was so disgusted with his bug body that he couldn't bear to touch himself. By this passage, he's not sure if he can distinguish between bug noises and human coughing anymore.
He was lying on his back as hard as armor plate, and when he lifted his head a little, he saw his vaulted brown belly, sectioned by arch-shaped ribs, to whose dome the cover, about to slide off completely, could barely cling. His many legs, pitifully thin compared with the size of the rest of him, were waving helplessly before his eyes. (1.1)
This passage accentuates Gregor's lack of control over his new body. The body itself is described as an impermeable thing, like a tank. Gregor can only look at his legs; he can't control their movements.
He felt a slight itching up on top of his belly […] found the itchy spot, studded with small white dots which he had no idea what to make of, and wanted to touch the spot with one of his legs but immediately pulled it back, for the contact sent a cold shiver through him. (1.4.)
It's ironic that Gregor can't stand touching himself, even to relieve an itch, when he experiences everything through his own skin, or rather, on the surface of his bug body.
Gregor was shocked to hear his own voice answering, unmistakably his own voice, true, but in which, as if from below, an insistent, distressed chirping intruded, which left the clarity of his words intact only for a moment really, before so badly garbling them as they carried that no one could be sure if he had heard right. (1.7)
Yet another instance where Gregor feels himself split between his bug side and his human side. We normally take our voices for granted as an expression of ourselves, but Gregor doesn't have this luxury. With his voice reduced to a "distressed chirping," Gregor loses a significant means of communicating to, and connecting with, other human beings.
[T]he pads on the bottom of his little legs exuded a little sticky substance […] his jaws, of course, were very strong; with their help he actually got the key moving and paid no attention to the fact that he was undoubtedly hurting himself in some way, for a brown liquid came out of his mouth, flowed over the key, and dripped onto the floor. (1.24)
Gregor's imperviousness to pain brings up an interesting question about his consciousness. Is he impervious to pain because there's a disconnect between his bug body and his human brain which prevents his human brain from processing bug sensory data? Or is he unaware of the pain because animals don't have consciousnesses – they feel pain without thinking, "Hey, I feel pain" – and thus he's losing his human consciousness?
"Human beings have to have their sleep." (1.5)
Is Kafka suggesting here that Gregor turned into a bug because he didn't get enough sleep? That's a good one to try on your parents or anyone else the next time they try to wake you up early. In any case, you'll find Gregor making all kinds of statements in the story that are ironic because of the context. That is, what would normally be a obvious statement – everyone agrees that human beings need sleep – becomes absurd in view of the fact that Gregor is a bug. (This, by the way, is what literary critics call "dramatic irony.")
He especially liked hanging from the ceiling; it was completely different from lying on the floor; one could breathe more freely; a faint swinging sensation went through the body; and in the almost happy absent-mindedness which Gregor felt up there, it could happen to his own surprise that he let go and plopped onto the floor. (2.20)
There are moments in the story when Gregor actually enjoys some of the things his new body can do. The ability to defy gravity and hang upside down from the ceiling – that's kind of nifty, isn't it? The down side of his new powers is that he literally loses his mind.
Now he really had no more time to examine the good intentions of the two women, whose existence, besides, he had almost forgotten, for they were so exhausted that they were working in silence, and one could hear only the heavy shuffling of their feet. (2.24)
It's interesting that the women here act like Gregor – see in particular the scene where he's moving refuse around his own room in 3.9. Like Gregor, they no longer talk; exhausted, they don't even walk normally, but "shuffle" their feet in the same way that Gregor shuffles around. It seems that their work has literally taken the life out of them – even reduced them to a bare, animal existence to the point where they become forgettable to Gregor. Similarly, Gregor's needs become more and more forgettable to them as time goes on.
Gregor's legs began whirring now that he was going to eat […] "Have I become less sensitive?" he thought, already sucking greedily at the cheese. (2.7)
Gregor's new body responds in strange new ways to his needs. (Imagine how awkward it would be if your legs spun around every time you got hungry.) Before you dismiss Gregor's habits as another indication of his disgusting vermin-hood, take a look at the way the boarders eat in 3.10-11.
Was he an animal, that music could move him so? He felt as if the way to the unknown nourishment he longed for were coming to light. (3.14)
It's unclear why musical taste should be indicative of Gregor's animal nature. Certainly there are mythic references to animals being swayed by music. Aren't humans equally enthralled by music? Perhaps Gregor is just noticing that he is moved by music in a way that he isn't moved by human language.