Ok, this is just for the family room so I'm only trying to make it the best I can with what I have to work with. The dedicated home theater area is already scoped out in the basement. :)

I'm using JBL in-walls all around. For the front wall the right speaker is going to be located on an external wall. So, I have a couple of questions.

I've been reading that insulation is good for in-walls. I'm concerned about safety though. There's already batting and vapor barrier on the right side but just empty walls on the center and left. So, if I stuff some batting into the cavity for the center and left that should help equalize the sound, right? I'm not looking for perfect, just not really badly unbalanced.

Is there a fire hazard having the rear of the speaker pressing on the insulation or the vapor barrier?

Any other suggestions to try to make this sound as good as possible, within the constraints I have?

Thanks!

Adding un-faced insulation to the stud bay has two effects. First it increases the effective size of the bay. That may improve or worsen the low end performance of the speaker. Second it will damp standing waves and that is a good thing. Foil faced insulation will decrease the size of the bay and will be less effective at damping standing waves.

Some makers specify a minimum volume for the bay holding the speaker. Foil faced and tightly packed insulation will decrease the effective volume.

If the speakers are set to small and the LFE cross-over is 80 Hz or more the low end changes may not be at all important. With that assumption insulating both stud bays might help and wouldn't hurt.

If the speaker has a hole in the center of the magnet assembly (many woofers do, most mids and tweeters do not), it helps cool the voice coil and should not be blocked (although it is not a fire hazard). Other than that there is no issue if the insulation touches the back.