I use Windows 7 on a Lenovo 7760 monitor with internal speakers, and up until recently, I've enjoyed almost crystal-clear sound. However, when I started up the computer one day, I got a message from my AVG antivirus software saying there was a threat that needed to be taken care of (I didn't pay attention to what the threat was), so naturally, I had AVG delete the threat, and when I tried to play a YouTube video, the sound volume was too low, and when I tried to raise the volume on my computer, the sound, not just in the YouTube videos, but everywhere else on my computer, was distorted/muffled-sounding and not at all crystal-clear like I was used to.

So, naturally, I tried everything I could think of, such as disable sound effects on my Realtek HD audio driver, select a higher audio bitrate, disable sound effects, install a new and better Realtek driver, install the official Microsoft audio driver, and some other things I forget but I'll remember if I've done them if they're suggested to me, but nothing seems to be working? Will I have to live with low-quality audio for the rest of my life, or is there a way to fix this?

EDIT: OK, I've just discovered that the problem with the sound on my computer is there's too much bass and too little treble, but I'm still at a loss as to how to fix it, any suggestions?