A few questions.
Did you put the board against the standoffs on the case or did you put a plastic washer between them ?
Are you sure you don't have a standoff in the wrong place ?
Have you removed the sound card and used the onboard sound, if the sound is coming from the speaker. ?
Is the sound coming from case or from your sound in speakers ?
Does your CPU have graph, can you remove the card and use onboard graph.
Is the retail store a asrock store or repair shop, if not and you can't isolate the problem, you need to send it to asrock.
If the store really tested it, maybe your PSU is causing the problem, if they didn't hear a problem.