I need a extension cord with

500 for a quad box with a short cord and a plug are you nuts ? And noise and or sound altering from the cover… stupidity ! Please get an electrician put a quad box where the duplex is nobody in their right mind will notice the difference, you’re talking inches of drywall altered. Unless a person is using high draw, high output amplifier’s, there won’t be a problem ! The first question should be what type of equipment and demand is predicted.

@curiousjim   Get yourself over to Home Depot and buy a quad outlet box, a couple good outlets, a few feet of 10 awg wire and a 20 amp plug.  If you're worried about a metal top plate sucking the sound out of the music, buy a plastic one.  The whole thing should cost you less than $20.  Try it out before you spend $500 on something else.  You know how your system sounds now and should be able to tell if it still sounds the same.  Gamble the $20 and report back with your findings.  We are always preaching on this site to "use your ears".  I suggest you follow that advice here.  Cheers

Furman SS-6B-PRO 6-outlet Pro Surge Suppressor Strip ...look at the reviews. I have 3 of them. You are extended and protected.

Here's a video that shows how to convert a single wall outlet to two or more. If you are handy with tools and comfortable with wiring, it's really not that big or difficult of a job for a DIY project.