Sub D

The warning "No signal Entering the power saving mode shortly" means that the monitor is not receiving any display information from your computer.  This is normal when the computer is off or "asleep."  Typically, you will only see this message for a brief time, because as soon as the monitor senses that the computer is not providing the video signal, the monitor itself will go into a "sleep" or power saving mode.

Although the display is blank when the monitor is in power saving mode, the monitor's electronics are not fully turned off.  You should see the power button on the bottom edge of the monitor (towards the right side) blinking. 

When you restart the computer, the display output signal from the computer turns off for a brief period.  If the monitor detects this loss of signal, it will go into power saving mode, displaying the warning message first.  If your computer goes through the restart process quickly enough, its display output signal may return before the monitor has a chance to go into its power saving mode.

Your monitor has 3 different inputs:  a Sub-D input, an HDMI input, and a DVI-D input.  The Sub-D or VGA connection is an "analog" signal and is what computers have used for display output until relatively recently.  DVI-D is a digital video signal.  HDMI combines digital video and digital audio.  Most newer computers use this type of connection.

When the monitor displays the picture of the Sub-D connection, it is letting you know that it had been receiving a video signal on that input and has now detected that the signal is no longer there. This would be useful if you had several devices connected to the monitor, so you would know which one had the loss of signal.

If you see the warning message when the computer is NOT in the process of starting, restarting, or powered down, it almost certainly means that there is a problem with the cable.  Sometimes, that just means that the connector on one end or the other is loose, but if you've checked these connectors, the problem may be with the cable itself.  If you "wiggle" the cable a bit, does the warning appear?

I can't help as far as the sound is concerned.  It's probably coming from the computer rather than the monitor, but without hearing it and without knowing what the computer is doing when the sound occurs, there's little I can suggest.  Where did you "go into settings" to turn off listed sounds?

I will try to finf the user manual tomorrow. The connector looks like blue one. 

The warning seems to be sporadic & random. 

Thanks!

I indicated Windows 10.

I have moved your question to the Windows 10 forums.

Does your monitor show your display correctly?  That is, does it look fuzzy, distorted, the wrong size or wrong colors?

Does the connector at either end of the cable between the monitor and the computer look like the first image (which is a "Sub-D" connector for VGA) or the second image (which is an HDMI connector)?  There could be other types, but these are the most common.

Under what circumstances does the monitor show the image and make the sound?

This seems to be some sort of warning generated by your monitor.  Look in the User Guide for your monitor or please provide the specific model of your LG monitor and I'll try to find it for you.

The Sub D connector is a VGA connection.  The VGA signal from the computer has no sound component.

Describe your system and monitor in detail.  For example, how old is your monitor and PC?