your problem is very simple.

when you output through HDMI from your xbox, the audio signal is DIGITAL.
your headphone will not be able to get anything as it runs on ANALOGUE.

if your tv is able to convert the DIGITAL audio to ANALOGUE audio then you will be able to plug in your headphone. if not you will need an external decoder.

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Added on May 14, 2011, 11:45 pmaddon : a decoder is a MUST if you wanna plug in your ANALOGUE headphones to use DIGITAL audio. no simple converter or adapter will help you. you need a DECODER.

This post has been edited by swhung: May 14 2011, 11:45 PM