Is the AR15 significantly louder with a muzzle break than without? Thanks.

Yes, as are all firearms.

It is SIGNIFICANTLY louder with a muzzle brake.  Also, those to the left and right of you will get a jet wash each shot.  Don't get me wrong, I LOVE my ak-shorty.  But it is louder than others, by a far sight.

I was at the range with a BM Dissipator w/AK brake, and a BM 16" w/o brake.  A shooter 6 benches down from me asked if the Dissipator was a .308.

ph119 is also correct about people to your immediate right and left.

I was told a muzzle brake decreases accuracy, is this true.  I like the way they look, but cant find many positive reasons for having a brake.  Any thoughts

Muzzle brakes do decrease accuracy a tiny bit, as do flash suppressors. But unless you are shooting in long range 700m + competitions you won't notice the difference. It's not like your nice 1 1/2" group at 100m opens up all the way to 6". More like 1 1/2" to 1 3/4" - 2".

With the addition of an AK brake on my M4 upper, it has noticably less recoil and muzzle rise than my 20" HBar with no brake/suppressor. I'd say that makes the AK M-brake well worth it considering that the M4 is about half the wieght which normally always = more recoil. It is extremely easy for me to pop off back to back shots on my M4 without losing my sight picture. I was telling a friend, it "felt" like I was shooting a Ruger 10/22.

I've never stood next to my M4 to test the side to side volume. At least to me, behind the M-brake, the volume seems a little less than normal.

I've seen some interesting up and coming concepts in M-brakes/suppressors that are actually supposed to increase accuracy. They are still in the research phase.

Fine for the shooter, a bit punishing for the folks to the sides.

I've got a BM 16" carbine and the BM Shorty AK, so I get to test them side by side.

Noticably less muzzle rise with the Shorty AK, though the .223 doesn't have that much to begin with.