If a store is crowded, people tend to buy more if the sound system is playing a fast-paced song rather than a ballad.
If I'm in a store and I hear oonce-oonce techno music, I'm distracted. My mind is dedicating attention to the soundtrack instead of convincing myself that I need to buy this whatever-the-heck I'm looking at on the retail store's rack. Plus, I am not much of a fan of oonce-oonce techno, so now I'm a grumpy customer who doesn't want to be there.
Picture yourself going into Forever 21. Loud, chaotic shopping experience with clothes everywhere. Now picture yourself going into an upper-scale place like Nordstrom. Especially when there is live classical piano music streaming through the multiple floors. Doesn't your mind feel a bit happier? Lighter?
In case you don't want to click on the article link below, the "boost" in sales... from 460 stores polled... was only 8% thanks to the lively music. Doesn't seem like much of a boost to me. I'll continue to seek out the quieter stores to let my mind think.