I suspect the problem is linked to your video card. Odd, right?
A video card you add into a PCIe slot has no real way to get an audio signal from a mobo audio system. But so many people want to use the speakers in their monitors for sound, and it's really handy to have them fed via the HDMI cable coming to the monitor. The HDMI design allows this. So most video card makers now include an audio system on the video card specifically so that system can feed audio through the HDMI cable from that card to the monitor.
The "catch" here is that, while a computer may have more than one audio system in it (like yours, with systems on the mobo and on the video card), Windows can only use ONE of those systems at a time. Now, VERY often when you run the driver installation utilities that come with your video card, it also installs that card's audio system drivers AND changes the Windows setting so that Windows will use that system. Then the jacks on the back panel of your mobo no longer have any output signals!
You can keep using the system this way, and let sound come from your monitor's speakers if it has any. Or, you can tell Windows to use the mobo audio system instead and have sound available from your back panel and front panel jacks. Of course, that means you need your external speakers working, not the ones inside your monitor. To make the change, go into System and find the Sounds control panel. In there you'll find a tab that allows you to make separate selections for the Default Playback Device, the Default Recording Device, and the Default MIDI Device. The Default Playback Device is what you want to adjust. I expect it shows a Realtek system (on your mobo) and a second system on your video card. Choose the Realtek mobo system, confirm and back out. Now try your front and rear jacks for sound.