Sound Recorder: Seems to be recording, but playback via WMP is silent

I've never used Sound Recorder before. I want to use it to record a flute choir I play in, not something coming in from the computer.

When I try to record something, the green bar seems to indicate that it's detecting the sound.

I save the wma file. When I play it with WMP, there is complete silence.

My control panel sound device for recording shows only Microphone Realtek High Definition Audio Default Device, with a green check on it. In properties, I've set both microphone and microphone boost to the the highest levels.

On Skype, Echo / Sound Test Service comes back loud and clear.

Troubleshooting tool shows no problem.

Help!

I've also successfully used Windows 7 speech recognition, to figure out how to help a friend with MS.

I rebooted, and sound recorder now works. Both skype and windows speech recognition worked before I rebooted. Clearly no problem with the hardware. I can't guess why a simple app like sound recorder would get kerfuffled when skype and speech recognition were both ok.