I think it's rude to just download MP3s from the Internet so I buy them from Amazon. But I want to share them with my immediate family. I know that if it was a CD, I could just lend it to them, but lending MP3 doesn't really work (I can't be bothered deleting files every time and copying them back again). Is that allowed to duplicate the files to my brother's smartphone?
Check the exact license terms. I haven't checked Amazon's terms; if I download music from Apple's iTunes store, there is a license that allows me to store this music and play it from several Apple devices - I assume that Apple has negotiated a contract with the record companies so they can offer that license. Amazon may have a similar license. And of course copyright law may or may not give you certain permissions.