First off, I always install these first anywhere I'm doing mining, and I would recommend you do the same - super cheap and easy to install:
http://amzn.to/2BYcqAkThis will give you the ability to know if something you're doing is having an effect, beyond the door being difficult to open or close. You might also want to pick up two of them, and have one be outside to cold side, and the other be the difference between cold side and hot side. Then you'll not only be able to tell whether you're starving your intakes, but how balanced the two sides of the room are.
On your intake/exhaust, I'm going to assume those are round since you're giving a diameter, but you can easily use normal HVAC duct values to get an idea of what kind of CFM you're making available:
http://www.waptac.org/data/files/website_docs/training/standardized_curricula/curricula_resources/duct%20system%20sizes%20and%20airflow%20quick%20chart.pdfor
http://buildingincalifornia.com/wp-content/uploads/2014/03/duct-sizing-chart.pdfYour two inlets are probably giving you 1200 cfm - assuming 0.1 inch of pressure. For your fan to be moving 1000 cfm at 12" means it's probably a little over 2x (non-linear) that static pressure.
You have a free-air opening which buys you basically 1800 cfm, but that's assuming there's nothing blocking it like a filter - if you have a filter on it, it will drop things considerably.
The first design issue with this that I can see, and sounds like you've experienced is short-circuiting airflow - meaning the fan is actually pulling air in from your free-air because of it's proximity. Short of you creating some sort of wall between these, air is always going to travel the path of least resistance, and will get sucked in by the much higher pressure fan right next to it.
If possible, you'd probably be better off moving all the machines as close as possible to the vent, going to an HVAC place and getting a plenum made that takes all of the air from the back of the miners and funnels it directly into the 12x30 - this would eliminate hotspots and be more effective moving the air where it needs to go - out. If possible I would add another 12x30, and plenum out to both of them.
On the sounds issue, the easiest thing to do to correct for that is just make sure the path for the air isn't straight. When the fans are inline with the exhaust, you get the best airflow, but the best channel for the sound to travel. You can also put louvers on your exhaust, and while it will cut some airflow, it will cut sound more. Especially if you put another air gap and have louvers going the opposite direction.