Have had three new nanos all ended up doing same thing volume going down by its self. Apple has problem with these and knows its in moisture getting in some where( thinks its volume bottoms ) I cut off end to a charging cable and left in when running with my nano. Did not help same thing happened on third nano. Try taking dry toothbrush and cleaning charging port and head phone port, helps sometimes!
Apple needs to own up to problem just like they finally did on iPhone antenna / bumper problem. Class action suit would help on nano problem!