Why doesnt my headset work for the 360?

i went to personal setings and it is on headset and the volum is up and than on halo 3 i went to setting on the gam and changed it to headset...its me and my friend playing i see the lil thang over his name wen i speak in to the mic but it seems that it is muted or sumthing so plaese can som won head it is makin me verrry angry