The armory WMO is "Monastery_war.wmo" in WorldWmoAzerothBuildingsDungeonld_scarletmonastery.
His group come from 000 to 019.
For the ID you just have to extract the WMO and open it in 010Editor.
For the WMO it's just here
http://puu.sh/iI0tp/adb3f8ef02.png, take the correct hexa (or you can use the
WMO template and open the MOHD header.) Dont take the screen for the armory, it's not.
And for the group it's here
http://puu.sh/iI0zp/e7c7dac31b.png. (Reproduce for every parts of the group, but usually, the sound is mainly controled in the Root. Some ambiance change are sometimes dones in the group, not really more than that.) and ofc it's not the screen for the armory group. It's just a random WMO that I found to show you.
And if you don't know already : If you don't take the ID from the Template (I don't have any template for the WMO group btw) don't forget to invert the numbers.
=>
http://puu.sh/iI0Fc/5a64b49d69.png is becoming : 9754
Once it's done, go on a website like that :
http://www.statman.info/conversions/hexadecimal.html, paste your hexa result and hit "To decimal".
Here you have some work mate.
And I was assuming you were a totally newbie in WMOAreaTable, So don't take it badly if I did too much "details".
I'm not a pro neither, I'm just working hard on the understanding of WMOAreaTable, but at least if you're not going to create a new ambiance for a new custom WMO. You shouldn't have any problem.
Regards.