the device outputting the sound mutes the speakers automatically when it detects a device attached to the headphone port. not much you can do about that.
what you could do is if you have a sound system you can split the outputs and use one to the speakers and one to the wireless headphone transmitter.
eg if you hook up to a soundbar or 2.1 system via a single cable you could split that and use a pair of wireless stereo headphones. one cable to the speakers one to the headphone transmitter.
or if you had a surround system you could use a 5.1 wireless headset and do the same except with more cables.
i'm not sure what equipment you are connecting though so solutions may vary (you should really be more specific in what you're trying to connect and how.. eg a dvd player model xxxxxx, tv model xxxxxx and a pc model xxxxx in a room with a sound system xxxxxx)