Think it's an unfortunate side effect of how the audio layer changed since Vista (that whole OpenAL mess). No longer having that direct hardware link like we had with DS3D (EAX and such) has knackered things up over the years--noticed some on-board audio solutions stopped detecting when headphones are plugged in.

My XP box has an Asus Xonar DX PCI-E drop in card, and even though it no longer detects when I plug in headphones to auto switch, if I change it in its control panel it flips sound to the headphones for me, even in XIV. Maybe using the proprietary control panel is the key to making it switch (instead of the Windows Mixer or something). Might have to test that on the laptop sometime to see. That old XP box is an oddball in general though. By all rights, it should be dead and buried by now... but it just keeps on running everything I need it to, so I can't bring myself to chunk it yet.