No sound after installing new GPU

You can fix this with a setting in Windows. But first the why?

Most video cards these days also contain an audio chip. That is so that it can send sound on some of the lines of an HDMI output port to your monitor or TV that has its own speakers. VERY often when you install one of these cards and it also installs its device drivers, it installs the driver for that sound chip and then changes a setting in Windows to make it the default audio output device. Now, Windows can only use ONE audio output device at a time, so this action turns OFF windows' ability to use any sound system on your motherboard. Thus, those front and back jacks no longer have any signal coming out of them.

To Fix, go into Start (bottom left corner) ... Control Panel ... Sounds and Audio Devices and to the Audio tab group. There you will find a drop-down menu of all the audio output devices in your machine, and there will be at least two. One will be on your mobo, often made by Realtek. Another will be the new one on your video card. Choose the m0otherboard sound output device and tell Windows to make it the Default Sound Output device. Back out of these windows and test. You should have sound from your front and back panel jacks now.

Maybe go through the Realtek tools that is used to configure its outputs and inputs, and ensure that it is set with volume levels up near the top.

There is another thing I'll detail, in case it is involved. The Realtek system I have (and many are this way) is designed to allow you to re-assign certain output jacks to do different functions. But the way it works can cause your sound to disappear. So, with the Realtek system set as your Default Sound Playback device, follow this sequence.
1. Arrange your system so you can plug in and unplug speaker items from your back panel, and see your monitor screen and keyboard at the same time.
2. Unplug all the things on the back panel jacks for sound output, and any headphones plugged into the front jack. Now reboot your system so it realizes it has NO devices connected to output jacks.
3. Go into the Realtek system and set it for your speaker system - I will write as if you have a system like mine - 5.1 surround sound, plus headphones at the front panel.
4. Start with ONE speaker plug - say, the front left / right speakers. Plug that into its correct jack - usually green. Now check your monitor right away. It should pop up a little window saying that Realtek has detected something was plugged into that jack, and asking for what. It will suggest a default possibility, like the front left / right speakers. If you ignore that, the screen will disappear after a short time, and it will NOT use the suggestion it showed you. Instead, it will send out NOTHING on that jack! So, before the little window closes, answer it and tell it - even if only confirming its suggestion - exactly what you just plugged in.
5. On to the the next - say, the front middle / Subwoofer pair into the orange jack. Follow the same process - answer the little window.
6. Same again (if you have them) say, rear speaker pair into the grey jack, and answer the window.
7. Lastly, plug your headphones into the front and see if you need to answer for that new item, too.

Once this is complete, go to your Realtek configuration utility and use its test tools to output sounds to your speakers. Mine offers various moving sound patterns to check that all speakers are performing, and help you balance their volumes.

This process is necessary to get the mobo Realtek system to actually send sound out of its jacks. Maybe it will help your problem.

Here's another thought to consider.

On some mobos with built-in audio chips, there is an option in BIOS Setup for what happens when you plug headphones into the front panel. Some let you choose whether or not using headphones in the front automaticaly shuts off the rear speaker outputs.

But more pertinent to your situation: I realize now that, since you are using a "USB Speaker system", what it likely going on is that the system you talk about really is another and different sound output device. That is, when you tell Windows to use it (and it probably did set itself this way when installed), as your default sound output device, then NO other audio output is being used by Windows. Now, normally the front panel headphone jacks connect to a mobo header that gets its audio signals from the mobo's built-in system. But if Windows has been told NOT to use that, and use the USB system instead to send sound out through your speakers, then there will NOT be any signal from the mobo audio chip to the front panel.

Re-read my first post. You can check for this possibility. Go into Window's system to specify the Default Sound Output Device, and see if that is set to use your USB system. If so, change that to make the mobo Realtek system your default output device. THEN see whether the front panel headphone jacks work. While you're there, note another related item, which is relevant IF your kids are using the microphone of the headset. Just as you can set only one default sound output device, you also can set a default sound recording device separately, and this should be set to the mobo Realtek system if you want the front panel microphone jack to do that job.

I suspect that, when you installed the new GPU, it also installed its own audio system and made that the Windows Default Sound Output device. That was the start of your problems. After that, one thing you have done is re-installed all the audio device drivers. In doing that, you probably also reset all of them to their default settings, and that may have been why the old settings for your USB speaker system were changed and the front headphone jack no longer works as it used to. I suggest you connect the USB Speaker system and make it your default sound output device. After that is working, look through its settings carefully to see if it has something for enabling the front panel headphone jacks.

Wow! What a frustrating puzzle!

Can you post a picture of that? Probably rightclick the sound icon in the system tray and choose the playing devices.
Can use Imgur to post a picture here,use the "img" link.
Click "new post" ->choose "upload images" -> "browse" ->choose the picture and click "open" ->put cursor on the picture that you uploaded->choose "share links" and copy the img link.

Just to see for ourselves what is the default playing device.

I realize this is a few years old but I'm having the exact same issue. Tried it all and still super low volume.
I'm hoping by bringing this one back there may have been a fix since.