There are three Kamui in the show: Senketsu, Junketsu and Shinra-Kōketsu (Omnisilk Kōketsu).
Shinra-Kōketsu is on another level compared to other Kamui. However, Junketsu is the same as Senketsu, but he never talks. Is Senketsu different from other Kamui?
Senketsu is the only one Kamui that can talk or communicate. Senketsu can communicate only with Ryuuko. This is because Senketu was made by Dr. Matoi with some modification, i.e Dr. Matoi used Ryuuko's Central Nervous System's DNA with Senketu's life fibers, which is why Ryuuko only can communicate with Senketsu and wear it.
After the end of the battle in episode 17, around 6:00, there is a conversation between Ryuuko Matoi and Aikuro Mikisugi:
A : Dr. Matoi made further improvements and completed Kamui Senketsu. Senketsu's life fiber have been spliced with your central nervous system's DNA
R : What ?
A : That is why he can only communicate with and worn by you.
A is Aikuro Mikisugi, R is Matoi Ryuuko. The exact phrasing might depend on the subtitle set.
Ryuuko's Central Nervous System's DNA is the main reason why Senketsu can talk but other Kamui cannot.
But Satsuki was able to wear it and use it. The reason may be that they are siblings. Satsuki can hear and communicate with Senketsu. They are siblings so their DNA match to some extent, and Senketsu contains Ryuuko's DNA. This may be the reason but this is more what I think than a confirmation of existing information.
There might be many reasons:
First, Senketsu was created specially for Ryuuko. This might have created link, that allows them to understand each other. It is clear that Senketsu doesn't really "talk" using his mouth. It is more like telepathy. This telepathic link might have been created either intentionally or accidentaly when Senketsu was made.
Second, there is question of relationships. Ryuuko and Senketsu are partners or friends. For them, it is symbiotic relationship. Satsuki on the other hand uses her willpower to dominate Junketsu so it can be used as a tool. And as such, there was no reason for her to listen to it, so even if Junketsu did talk, Satsuki would simply make it shut up, because Junketsu talking isn't what she need. Ryuuko and Senketsu were partners so it was obvious they would talk to each other.
And last is simply about production: The creators simply didn't think of Junketsu as character. And as such, they didn't give it any voice. Senketsu was individual character from early start and as such it made sense to make him talk. And as such, he helped character development of Ryuuko. Junketsu on the other side was just something used by Satsuki.
In my opinion, Senketsu is the only one that can talk because it was specially made for Ryuko and probably only people like Ryuko and Ragyo who are part of Life-Fibers can wear them because they are in sync.
