In the middle of creating character profiles for
boku_no_koe, I became immensely annoyed with the translation of vol. 1. Compared to the Japanese version, there is quite a lot of swapping, or worse, removal of the suffixes. The scanlation is simply not trustworthy. I've seen -san swapped for -sensei, -san and -chan removed so no suffix is present. It is irritating because the suffixes work very well to determine relationships and moods, and the depth of those. So I had to cross-check all the info...
However, it seems as if "Kaza" is closer to Hosaka and Yumi than I thought as they actually don't use any suffix after his abbreviated name... Kaza does call Yumi "Kurokawa-san", though. And as for Kokubu, he is less respectful and not "-sensei-ing" Yumi as the translation suggests, which makes sense as Yumi calls Kokubu exactly that, Kokubu without -kun, or -chan. Hence the closeness.
Kazama seems to like Yumi a lot. He "will listen to anything Kurokawa will say" and tells him he is "in his heart". Heh. Kokubu, too, seems attached to Yumi: he entered the seiyuu business "out of admiration" for him.
On the other hand, Hosaka is called Hosaka-kun, and Yumi is just Yumi, or Yumi-chan. It seems mostly like pet-suffixes between very close friends.
On a different note Tanahashi is more powerful than I thought. He doesn't deal with the lesser seiyuu, only when Jouchi gets a main part, Tanahashi shows up to talk to him about it (no matter that he went to the studio when we first meet Jouchi - but that was possibly to take care of Sigma's money cow, Yumi). This means that Tanahashi has underlings, and his secretaries shows up a couple times. Due to the way offices are laid out in Japan (open, one room, usually with the manager on a podium), it is not possible to determine whether he's a very much higher-up or "just" a office manager. However, he seems to be able to decide by himself about (the obviously talented) Jouchi's career, as well as managing Yumi. The owner of Sigma? Managing director? Not a standard salaryman, I think, and that was surprising. I guess I just admired his sexy man-wearing-glasses-ness and hadn't reflected that he might be quite up there... Hmm.
Now I have to go check my copy of Book 2 and see whether Jouchi actually calls Yaogi "san" or not. I basing Yaogi's age and the level of their friendship on the fact that Jouchi says "san" and not "sempai" (as he does with Kokubu), but if that's not accurate, I should go back and double check the original Japanese.
Tanahashi...I was just reading the manga again, and I noticed that Jouchi was surprised to see him at the beginning of the Book 1, which means that he doesn't normally rate personal attention like that. I don't think that the owner would bother coming out to see a new seiyuu, but he's probably some sort higher-up, as you suggested, a managing director or division chief or something?
I also noticed that the blond male manager is talking to Kazama in the first book about clearing his schedule for an audition, but it's a woman who later talks to him about overworking, so I wonder if they are supervisor and underling or vice versa.