Hey!
INOX = Stainless steel..... Personally I am not sure why you would use any grade of stainless steel for an audio box, unless you were building it for an extended installation on the beach. Stainless steel is awful to work with. Engraving it is a special art. If you are laser-cutting, powder coating and then screen printing, mild steel is fine (but it is DIY so if you really want stainless...)
Mild steel... I would not put mild steel on my CNC machines (for the same reason as stainless - difficulty in machining, spindle speed, cutting forces, etc - and swarf killing bearings and ball screws).
But, mild steel laser cuts beautifully, so if the powder-coat and screen print thing is what you want (with the benefit of increased strength over aluminum) then great - and if you are getting 20 of something made, then it can be cost effective (laser cutters generally have a minimum fee that a single 19" panel would not come close to)
Galvanizing - lets differentiate between hot dip galvanizing, and zinc electroplating.........
Galvanizing usually refers to hot dip galvanizing - steel is dipped into molten zinc making a thick industrial coating. Paint does not take well to a thick coating of zinc. I have never seen a commercial electronic enclosure, audio or anything, hot dip galvanized. Lets leave galv to heavy industry.
But - electroplating (including zinc electroplating but also other metals like nickel and chrome) deposits a thin paintable coating that can look perfectly acceptable as is for metal work (for me everything except the front panel). this is the finish on most of your commercial enclosures that have a golden or silver shimmer to them. They can even add other colors. Can be bought in sheets, or for a nice uniform finish, fabricate your parts, then clean up (sand blast if you like) then have them zinc plated at the end.
Then there is Ali.... This is my preference because of its available anodized colors, machinability, paintability, etc. But.... the grade of ali that is nice because of these characteristics does not press / bend well. So this is why boxes with steel sides top and bottom (electroplated or painted) matched with aluminum front and back panels are a nice solution. Or in the 500 series world, electroplated steel sled and bracketry, with an anodized ali front.
My 20c..
Cheers,
Tim