The second speaker/Line Out is probably muted in the Volume Control Panel.
However, using the second speaker/line out will not automatically mute the speakers like the front panel headphone jack will. You will have to manually turn off the speakers when using the headphones in this configuration.
An alternate that many use is to buy an extension cable for the mic. Then you can use the front panel headphone jack (which will mute the speakers) and with the extension cable being in front of the PC you can connect your mike to that.
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