This sounds like a wetness issue.
If you have the headset where there are controls on the wire, if moisture gets inside there, you can see the nano act as if those buttons are getting pushed. The moisture completes the circuit that is normally completed only when you push the button. The result can be volume going up or down, the nano speaking tracks, skipping a song. Any commands you can do withe the headset buttons can get caused by moisture in the headset.
The other place this can happen is when moisture gets into the buttons actually on the nano. This is much worse, because the moisture is not inside your nano, not just inside your headset buttons. This can cause all kinds of strange behavior and it can kill your ipod. Water and electronics don't mix well!
If you don't have a headset with button controls on the wire, then you can be pretty sure that some moisture has gotten inside your nano. You will want to put it to sleep and let it dry out to prevent more damage.
If this is happening because you are sweating on it, you will want to figure out a place to wear the nano where you will likely sweat on it less. In my case, I stopped wearing it on my waist and started wearing it attached to the bottom of my shirt or sometimes clipped to my collar.
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