There is apparently a problem with the S4 and the Lollipop upgrade. Right after AT&T forced the upgrade on my phone the Volume decreases to minimum automatically. I can reset to maximum and after 10 minutes or so it is right back to minimum. Searching all of the forums shows that this is a major problem and there are no solutions. I spent an hour on Samsung tech support and there only comment was to wipe the phone and factory reset. Sorry doesn't look like much help. If you can take the phone back and get something else I would do so.
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