Sleep Training?!

Back when we lived in apartments (and even in our tiny condo), I would put the kids down for naps or bedtime with their bedroom door shut and a lullabye cd playing softly on repeat. Living in apartments, your space is limited; and with limited space, noise travels fast. I started the "closed door" rule for obvious reasons--so that it would block out whatever noise we were making. I noticed that that wasn't 100% working, so I started playing the lullabye cd on repeat so that there was some sort of constant noise in their room--so that if we laughed at a tv show or talked to eachother, it wouldn't startle the kids. 

Little did I know that having that music playing everytime they went to sleep would serve as the mother of all cues that it was time to go to sleep. Once they got used to it, as soon as I would turn that music on, they would fall right asleep. This turned into the most effective way to 1). get the kids to go to sleep, and 2). get them to stay asleep. I never had to try a "cry it out" method because this solved it for me.

And that brings us to now. Maverick is 4 1/2 years old and still naps and goes to bed with his music on. We are now in a big house so noise traveling doesn't seem to be an issue; he now sleeps with his door cracked instead of fully closed. He wants to sleep with it fully opened, but I don't know if I'm ready to do that yet. Madison still sleeps with her door closed because she would guaranteed leave her room if that door was open. Maverick, however, knows that bedtime is bedtime--and he won't leave his room. 

How do your kids sleep? Door opened? Cracked? Closed? I understand that this is totally not a big deal--I'm just curious to see how your guys' kids sleep. What methods, other than 'cry it out,' did you have success with? I have so many friends and family that are delivering their babies now and over the next couple of months--I'm sure this info could help them out! :)