In Control Panel, dig out the "Sounds and Audio Devices" option, check the "Audio" tab, and make a note of what the "MIDI music playback" option is set to.

Try those sites one at time, check MIDI playback after each, and see what this option is set to if MIDI playback stops again. Chances are it's changing this, so change it back and things should work again. Then write a nice little email explaining what they've done and how it would be very nice if they stopped doing it. Alternatively you could write a thoroughly abusive email, bcos they'll deserve it...

Graham.

PS. Note that I only suggest intentionally breaking this because you've already demonstrated that you can revert to a working version! :-)