Really weird... I use my phone a lot to listen to mp3's with my Shure earphones and never had any complaints. So I just tested it again with the standard HTC earphones and I have to say there still are no problems here. Actually if I turn the volume all the way up it's just too loud to listen to without getting any eardamage
My phone is unbranded and i'm not using any 'gain software'... So maybe it is linked to T-mobile, seems a bit weird though.
Cheers