Your best bet would be to buy cable that is single strand then twist it yourself to a length you like. I would probably use two plugs and keep the y adapter because the separate plug for mic and sound is more standard and will work on a PC if you forget the mixamp.
I like this wire http://stores.ebay.com/Johns-Silver-Teflon-Wire-Shop/26-AWG-/_i.html?_fsub=3966783&_sid=155009&_trksid=p4634.c0.m322 You might want some different colors.
http://cgi.ebay.com/BLACK-550-LB-TYPE-III-CORD-PARACHUTE-PARACORD-100-FT-/370505897753?pt=LH_DefaultDomain_0&hash=item5643df5719#ht_1458wt_1064 <-- some paracord to cover the wire
Then I would use 4 cables for the sound in a litz braid http://www.chimeralabs.com/diy_braid.html cover them with the paracord.
Then I would twist the two cables for the mic together and also put them in paracord.
Finally twist the audio and mic cords together and heatshrink them near the bottom and where they enter the headphone, and use a zip tie inside the headphone cup to keep the cord from pulling out.
That is how I would do it. Also for the audio you want a 3.5mm plug that is stereo and for the mic you want a mono 3.5mm ( I like switchcraft plugs but rean is also good).
edit:
or you can do a 6 wire braid http://seriss.com/opcs/install/braid.html
then just split the mic and audio 2-3 inches away from where you are going to put the plugs on, and put some heatshrink on that spot (it will look like a y split there are various methods of making this look good).
Edited by Danthrax - 5/6/11 at 3:48pm