After reading all the posts and re-reading the OP's initial post, I think I understand what he wants to do.
The headphones claim to have a range of 300ft which hopefully is enough range for him to be in another room listening to music being played on his desktop. He wants to be able to control the music playing on his desktop from his laptop.
One way I can think for him to be able to do that is to use a remote connection to remote into his desktop from his laptop and then he would be able to change songs, pause, stop/start, etc... the music being played on his desktop from his laptop. As he said, use the laptop as a remote control. He could install TeamViewer on both computers to be able to do that. Or, if his desktop is running a "Pro" version of Windows, he could use Remote Desktop Connection.
Having said that, it seems like kind of a hassle just to use those headphones to listen to music in a different room. Personally, I would rather have headphones that plug into the laptop and then have my music saved on the laptop (more portable) or at least shared over the network from the desktop.
Another way to use use the headphones would be to plug the transmitter into the laptop and play the music on the laptop (leaving the desktop out of the mix) but that would require carrying the transmitter from room to room and plugging it into the mains there.
For the record, I have a pair of those headphones but I only use them to listen to TV late at night when my wife is asleep and use them only in the same room with the TV. They work great for that.
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hdmi - There is no place on the headphones to plug in the cable you suggested. The transmitter can be connected via RCA phono plugs or 3.5mm plug but the headphones themselves are wireless only.