Sometimes the player's voice is quiet, I want to change the volume for each player like Discord.

I don't know about the latest update but before you can do that in the social menu. (the mojang default key is P)

Yes. You can also hold RBM while looking at the player, and then use the scroll wheel to adjust the volume. It's very convenient.

We really should update the mod description, and also the wiki, since there is a lot of features that aren't mentioned there. I will start working on it today.

I don't know about the latest update but before you can do that in the social menu.
(the mojang default key is P)

I did not know that. Wonderful!!!!!!

Is there any documentation about it? If not, I would be grateful if you could add it.