I hate to bump such an ancient thread, but I figured I might as well add my experience with this particular issue.
I purchased my STX in early 2012 and I have experienced this same issue approx. 4 times. I am pretty sure I experienced this issue on both my headphones as well as my speakers, so I am leaning away from the headphone amp on the card and towards a hard-to-replicate driver issue.
I am running Windows 8.1 and had my playback settings at 44.1KHz/24-bit. I am currently using the 1.72a rev 2 version of the UNi Xonar drivers, but since I've had this issue a few times over the past several years, my version of Windows as well as the driver versions have changed. I still get the issue.
I ran into it most recently (two days ago) while browsing Youtube in Firefox. I was using the HTML5 player and seeking through a random video. As soon as I let go of the mouse on the seek bar, I immediately went from normal playback to massive distortion. After tearing off my headphones and stopping all audio playback, I had the familiar tone coming from my headphones until I rebooted my computer.
I haven't had any issues since. Given how infrequently this has occurred in the past for me (about once per year), any testing/troubleshooting is nearly impossible.
I don't think it's a hardware incompatibility because I regularly swap components out of my rig so my PC is a completely different machine than it was in 2012.
It's annoying, because I cringe everytime I pull my headphones on now.