I get how you may be feeling because I'm passionate and speak loudly too. I do worry about this occasionally but I don't worry about volume in therapy. Then again, my therapist works alone so we're not bothering anyone. I've only once ever been in a therapy session in a multi-office setting as a client and I didn't worry about this then, probably because I wasn't hearing anyone and I just assumed they weren't hearing me either.

It's a tough situation, I think. You shouldn't have to censor yourself in therapy and at the same time I understand that everyone else in the building also needs silence. Perhaps if you are worried, you could check with your therapist what s/he thinks about this, perhaps ask for feedback if you're really talking that loud and if it might be an issue. At the very least, if it causes problems, you could use this as an opportunity to learn to have a context-appropriate speaking volume (and get loud elsewhere), or best case scenario perhaps you'll be told that it's okay, the building has very solid walls or something of the sort.

Anyway, I appreciate your thoughtfulness in asking yourself this question.