Best rack that can be built into the wall

Check out Parts Express (www.partsexpress.com). They have some rack mount types that are reasonable. Also Atlantic Technologies makes a very nice custom faceplate rack. Cosmetically, it's great, but acoustically I don't think it's tops--it's also rather expensive. Personally, I would go with the parts express, and modify the shelving with damping and isolation devices.

Check out AVRak.com. I plannning to got one of these.

Rivercity> Check out the Standesign as mentioned by Tigger. It may suit your needs. However if you are putting this in a wall where you will have access to the back from another room, you could probably get a local custom cabinet shop to build it. It would be very simple to build since you only need sides, shelves, shelf standards and maybe doors.
Build it with wood. Veneered plywood is ok and comes in many species ie oak, birch, cherry, etc. If you must, use solid wood. DO NOT use MDF. It is not structurally sound and was never intended to hold any weight. I speak from 30 years in the woodworking business. If you are concerned about vibrations use isolation devices ie sorbothane. If you need further assistance feel free to email me.

I don't know that they are "the best", but I just saw the neatest stand that would be perfect for just that. They are stand design stands www.standesign.com what's so great about these stands is you can hook this metal attachment onto the front of the stands so that they can roll out. They even have built a in cable management system. they are a modular design, so they are completely customizable. The only weak point that was apparent to me were the shelves themselves, which were made out of cheap wood(not even sure if it was MDF board) so those would need to be swapped out (which isn't a proplem as they are totally removable.

As to the actual "sound" of the rack, I can't say I havn't heard them in a system. My boss actually ordered two of them so I hope to know shortly. Anyway this is a long winded post, go check them out on there website.