vibrate instead of ding on plug in

This is trivial but driving uscrazy!  I just bought 3 new iPhone 5s.  All identical all running the same software.  We also have 2 others bought 2 months ago.  When you plug in to charge your phone, it makes a "ding" like when you get mail.

BUT inexplicably one of the new phones does not do this, but instead vibrates.  It is beyond annoying to the person whose phone this is.  She wants to hear the "ding" - she is my boss.

I have compared all the sound settings and ring settings and I see NOTHING any different on these.  They all seem to be set the same.  It is not on airplane mode, etc.

Does anyone have any idea why one phone does this while the others do not?  or how to set it to ding when plugged in and not vibrate?  there's no choice of "charging" under Sound choices.

Thanks!