I used to have a headset like that and I’m pretty sure that middle button was a play/pause for audio. Usually it coinsides with your windows media or primary audio player.
KC1CPS

> On Jul 17, 2015, at 9:04 AM, David Pearson <[log in to unmask]> wrote:
> 
> Hello List:
> 
> I have a Logitech headset(possibly model #390), but not positive.  It's 
> plugged into an available USB port located on the front of the pc tower, and 
> seems to be functioning.
> 
> There is a plastic block along the headset wire(one side of which is 
> smooth).  On the other side of this block there seems to be a smooth button 
> surrounded by a plastic ring.  Pushing on this plastic ring seems to 
> raise/lower the headset volume level(depending on which side of 
> ring(top/bottom) is pushed, but I can't figure out what the smooth feelling 
> button in the center of this volume ring is for.  No documentation came with 
> this headset, and I'd appreaciate it if someone could "enlighten" me as to 
> its purpose.
> 
> Sincerely yours,
> 
> 
> 
> David S. Pearson-wa4dsp 
> 
> 
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