imac headphone jack/speaker switching

hi- have 9,1 imac around 2009 , has had problem time time, thought should address sooner or later.

specifically, sound tends lost when plug headphones in. have plug , unplug, nothing works @ today--i have no sound right through speakers or headphones. no volume error sign in either case when use buttons- goes , down no blip blip noise.

known issue cure? snow leopard , lion both have given me same issue. i've read other posts on , haven't been helpful. system prefs sound says internal speakers.

 

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