Surprisingly the Xbox 360e has a 3.5mm headset jack on the back... but it is listed as AV/component on spec docs. Mine even came with the AV cables shown below (composite video & stereo - red, yellow, white), using this port. I wanted to test out my existing 3.5mm headphones for game audio (versus chat only via the controller port). We likely all have a pair of these headphones around the house.
To my surprise... while using HDMI out for video, the 3.5 jack will still output an audio signal to a pair of 3.5 mm headphones. Unfortunately, I had no volume control. I am assuming when designing, they were thinking it would be feeding into an audio receiver with volume controls. Of course my headset cord was also WAY too short to reach the couch.
I set out on a cheap way to properly... well working way... to connect a pair of existing 3.5mm headphones with volume control. Keep in mind you'll need to turn down your TVs built in speakers.
I went with a KabelDirekt 10 foot 3.5mm male to 3.5mm female + Inline Volume Control for 3.5MM Headphones + Exiting Pair of 3.5mm noise canceling headphones.
$10.19 + $6.99 = $17.88 shipped on Amazon. Can't beat under $20 bucks.
Now I can crank up... or down... some Halo 3 without worrying about being too loud. It also reminds me of the glorious headset jack on the model 1 Genesis!
EDIT/ADD: I highly recommend you check out the thread below where I ask the RB resident audiophiles to chime in. They recommend amps to better get this set up done. We also discussed accomplishing this on ALL retro consoles with RCA cables:
Audiophile Help - 3.5mm Headphone on ALL Retro Consoles