I also like the sign idea. I think you could take it a step further.

During a team meeting, or via memo, or small gathering, tell the co-workers the idea. However, ask them if they feel a certain day, or time of day would be best.

"I am thinking of trying to keep the recording time the same as much as I can week to week. Do you think Mon - Wed - Fri is best, or what about in the mornings, from 9-11? Also, as an aid, I will hang a sign on the door, so everyone will know when I am recording. We can't have the company recordings having random laughter or sounds in the back ground or it won't look good for us".

Something like that, in your own words. So the team feels that it is a group effort, and they have some control over losing control of their choice to speak.

If that doesn't work, you will have to go to the boss and say, "random office noises are being picked up and you don't want it to be unprofessional, so is there a quieter area they could think of."

Good luck to you.