I am using Windows 8. I have a set of speakers that are connected through the headphone jack and I also have a Projector that is plugged in through a USB Port. Whenever I plug the projector in, I can't seem to get the audio to come out of the headphone jack anymore. When I look at the playback devices menu, the speakers for the computer and the headphone jack just disappear. Please assist.
As far as switching back to the headphone jack once your projector is plugged in, depending on the make and model of your projector, you may have to go to device manager or playback devices and disable the usb audio device that the projector uses, unplug the projector and plug it back in as it is "disabling" the headphone playback device. (disabled sound devices do not appear in "playback devices" in windows sound settings.)
Please refer to the first link for some analog options for splitting sound output, or consider using Virtual Audio Cable to create a device that will do the outputs that you want.
I discovered this the hard way when I couldn't output the sound from My Windows 8 HTPC through speakers and SPDIF to my HDTV at the same time.