Why does Windows Mobile changes to vibrate automatically?

[AncientPC]

I have an HTC Titan running WM 6.1 (with nueExtROM), but had this problem with Sprint ROM as well.

I use the HTC Home Plugin with the Normal sound profile. However, when my phone is idle for a long time it will switch to vibration mode automatically. This is really annoying because I'll miss calls when the phone's on the charger in another room. If I switch back to the sound profile tab, Normal is still selected. How do I prevent my phone from going into vibration mode automatically?

Also, what does the Automatic sound profile do?