In your Realtek app, set it to 5.1. Do a speaker sound check from the app to see if all the speakers are working one at a time to eliminate cabling problems. You may have a bad wire for the rears so test it first using the app. Once all is working then move on to the real world test.
Now when playing a movie that is 5.1, it's using dolby digital or DTS. The rears are naturally low. Also with a computer the fronts are in your face next to the monitor, and the rears are way back. You need to crank the volume of the backs and reduce the volume of the front speakers wither with the speakers themselves if possible, or in the Realtek app. Adjust the levels so the rears are louder and balance things out to your liking.
Once...