If you are using an HDMI cable to connect your PC to your projector the PC will sometimes detect the projector's speaker and set it as the audio device. To change it back to the PC speakers just right click on the speaker icon in the system tray (lower right icons on your desktop) and select the PC speakers.
Watch a quick video of the process below.
https://www.dropbox.com/s/v5vyv67nb3gqyg4/Change%20audio%20from%20projector%20to%20speakers.mp4?dl=0