For the audio gear, the items that get hot should have the most ventilation, so if you have a hefty receiver or external amplifier, that will want a few inches above it.
There are plenty of threads on placing any speakers inside another cabinet. Pretty much not a good idea. If you have to do it, I would go with a speaker cover type material for the front of that cabinet door and let it fire out the front. You will most likely want a sealed sub as well.