Instead of replying to older posts i'll just create another one. I've seen few people having issues with low volume with Realtek HD, I was one of them. My sound "system" is just pair of headphones so 100% volume was not enough for me. So after few days of messing about I found an solution:
1. Open Realtek HD Audio Manager from taskbar
2. Under ANALOG
you should have inputs, right click the green input (Depends if you have it plugged on front or back) and choose Connector Retasking
3. Check the option "Headphone"
Now the volume should be louder.
What also helps is if you check Loudness Equalization under the sound effects tab in Realtek HD Audio Manager or Speakers Properties.
I hope this thread helped.