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First, use alsamixer and see if the headphones are in mute mode. [MM]= mute. To umute , select the channel (navigate with left/right arrow keys) and click the M key on your keyboard. To increase/decrease volume, use up/down arrow keys. Exit from alsamixer with Esc key.

To invoke alsamixer, open a terminal and write

alsamixer

If this doesn't help, install pavucontrol ( sudo yum install pavucontrol ) and try to control the device from there. See if it's muted or any other info.