I experienced the same problem when I upgraded windows 8 to 8.1. The videos that I was watching online that used flash player had garbled or muffled speech. After some troubleshooting I found the problem and a simple solution. I went to the Realtek HD Manager and the Speaker Configuration was set to 7.1 Speaker, I changed it to Stereo, and that fixed the sound problem. The update must have changed my default preference. I believe Speaker Configuration of 7.1 Speaker is for several speakers and a subwoofer. I supose the subwoofer setting must of produced the muffled sound. On your desktop just go to the Realtek speaker icon, right click it, and choose Sound Manager. Then check your Speaker Configuration and make sure it is set to Stereo.