Yet another interesting change in the way Vista operations deals with the line in / microphone / various other possible audio input sources. Vista apparently uses a software level driver to adjust itself for the source of the input. One of the side effects of this is that the sound coming in from that source will not play through the speakers or headphone jack by default.
I say by default, but it is a little more complicated than that. In Windows XP there was a setting in the playback device properties that allowed you to monitor the input, effectively playing the sound from the line in through the speakers or headphones. A quick look in Vista shows that this option is missing.
Thanks to this post which pointed me to the Dell Community website, I was able to modify the registry. My screen looks slightly different than the .NET Devhammer site, but the result is the same.
[EDIT]: Dell changed their community site around and I can no longer find the thread from above directly. Thanks to Google’s cache I’ve tracked it down again. The key you need to find is:
(note: there are many entries with that GUID, you will have to look through each of them until you find one with the 000\Settings\Filter\SpeakerHp sub key)
You just need to create a BinaryValue name EnableInputMonitor and set the value to 01.[/EDIT]
One word of warning, after making this change you may experience serious feedback if you use a headset with a built in microphone. I found that I must turn off the monitor function before plugging my headset in, otherwise it produces ear-splitting feedback that can be heard throughout my home. I had not realized that headset could produce that many decibels!