Some alerts not playing.

[ExAm]

Just the other day, I discovered that some of my alerts weren't playing. Instead, I got a low pitched beep from my motherboard beeper. I checked the control panel, and nothing seemed to be out of the ordinary. Any ideas?

[TPE]

Usually means there is no audio device installed. Check and see if its under device manager.

[ExAm]

I can hear everything else, just not one or two alerts from Thunderbird and one other app.