...and I would tell you that the projectors you are using are horrendously under-powered for your setup.

Look, if this is all budget based, then you have to do your own leg work to determine if what you have is enough. The screens are almost 150 square feet, and THX calls for about 16-18 lumens per square foot to fill any screen. 16 times 150 is 2,400 lumens. So if the projectors can actually deliver that much light output, which is extremely unlikely, then you are in the area you need to be.

But you are using 4:3 projectors, with 16:9 screens.

XR-30X - 1024x768 resolution - 2300 Lumens (advertised) - 2007 model
http://www.projectorcentral.com/Sharp-XR-30X.htm

SP-820 - 1024x768 resolution - 4000 Lumens (advertised) - 2007 model
http://www.projectorcentral.com/BenQ-SP820.htm

Now, you say you 'tested' the Sharp, but did you test it at the size you intend to use it at, in the conditions you intend to use it in? How about the BenQ? You throw out 33% of the light when you go to 16:9 because you are only working with 1024x576 resolution. That's about half a million pixels, and realistically, about 500-800 lumens you may have available. About 1/3rd of what is recommended as a dark movie theater standard. With 1080p as the current standard, you also have about 1/4 the pixels available to you, and are using extremely outdated business class models which won't help image quality at all.

For sound? I think that you haven't mentioned what any of your gear is. 600 watts of power? From what gear? What is the speaker efficiency rating? How much audio does it actually deliver? What do you expect from the sound?

Watts are a measurement of power, not of sound. It's hard to have to much audio, but getting a couple of good quality commercial loudspeakers is certainly a good thing to have. Adding subwoofers is nice if it is possible, but building a nightclub outdoors is very pricey due to dispersion of the sound. You also have to consider who else may be impacted by the sound and what permitting may be required for that sound.

I would start at the start. Take what you have outside. Throw an image as large as you intend to use (16' wide!) on a wall, on a bed sheet, on something white. See how it looks. Feed it actual video material from your laptop or whatever you intend to use, and connect that to the speaker.

See how it sounds and listen to how it looks.

If you are happy, then that's all that matters.

But, if your test was a 100" diagonal image in your home, then that's not going to cut it when you want an image that's over four times the size.