Windows – Sound management when using multiple displays

I have a Lenovo AIO computer which is connected by HDMI cable to a TV as a second display. The two displays work perfectly visually and have done for months, but I am having problems with the audio. Somewhere along the line the sound output was changed (by my son) from being played through the second display (TV) to only play through the main AIO computer speakers system. This has worked fine but now I need to change this back so that the computer plays its own sounds through the computer speakers and the second display monitor (TV) plays its sounds through its own speakers.