+1 on what Bigus said, I'd like to add some food for thought.
First, have you already bought the in-wall speakers and ran wires to their location, or is this a planning stage thing and you are looking for information?
There is a sticky in this forum called Setting Up Your Home Theater Audio 101
, There is a speaker layout specific portion with a good easy readAudioholics - Speaker Placement: Setup Tips for Upgraded Home Theater Systems
Step 1: Center Speaker Positioning
First stick two rubber feet on the bottom front of your center speaker. Then place the pie shaped doorstops under the back of the center so the back is angled down toward the listening position. Next, place the laser pointer on the top of the center speaker pointed at the listening position. Then aim and adjust the beam of the laser pointer so it is just above the ear level at the listening position. Adjust the rubber door stops until the correct height is obtained. This effectively aims the tweeter at the listening position between your ears.
Step 2: Main Speakers Positioning
Position your front main speakers at least a foot off each back and side wall and you are close to the "Golden Triangle Rule" ( Example: speakers 8ft apart from listening position and 8 ft back). Make sure that the speakers are the same distance off the back wall with the tape measure, then place the laser pointer on the inside panel of the speaker enclosure at the height of the tweeter.
With the laser pointer beam active, rotate the speaker inward until the laser pointer beam is about 6" away (outside) from the center of your listening position. This will effectively toe in the speaker to a close position according to the dispersion patterns of your speakers. If your speakers have an unusually wide dispersion pattern, you may wish to experiment with the degree of toe in for optimal performance.
Speakers need to either be aimed at the MLP seats as described above, or if you have a AT screen placed behind the screen at appropriate location also considering the above.
Key is Front R/C/L should be close to ear level.
This thread at audioholics shows for "flat" screen HDTV not a big projection screen.In-Wall Speaker Selection & Installation Basics
Ideally the three front speakers (Left, Center, Right) should be placed at ear height (seated position)
IMO, your monoprice 7604 speakers are the wrong ones for your painted wall screen, they could go behind a AT screen, but are better suited for flat panel HDTV's.
Why sacrifice your HT audio experience after all the hard work you've done? (will do?)
Best thing would be to return them and consider in room speakers.
You'd still have the center located below the screen, but then could build/buy a center stand to angle the speaker at you, similar to what I did.
If you are really-really fixed on in-wall, then I'd consider to make the wall construction such that you could angle the sides towards the MLP and the center up, or look at other in-wall speakers that do that for you already.
I've not seen others do that, but there are so many threads here possible others have done it.
(Yea, some will say those in-wall speakers were not designed to be angled, there will be edge diffraction issues if you do that etc, hey I'm trying to give you information and options here)