i made this same adapter, but used an extra female 3.5 socket. it sounds great, there is a guide to do this here:
http://www.tracyandmatt.co.uk/blogs/index.php/2006/08/18/orange_spv_m3100_headphone_adapter_hack
make sure you solder it really good, or it will break the connection eventually, and the sound wont be good.