I did not mean to turn the screen (HMD) off and on. I mean check the default sound device on your PC..
If the sound was playing on your 4K HMD and then suddenly stops, look at your PC monitor (bottom right on the task bar) and click the sound icon
then choose the correct audio device. (4k HMD if you are using 4K speakers) or whatever your normal audio device is speakers.. headphones..etc
i have noticed that on some Piplay drivers (especially the last one) that the default audio device changes to another device on its own when using it or even before using it!
hope this helps