Sound Issues with Firefox 13 and Flash 11
This issue has been confirmed on two computers now. When using Firefox 13.0 with Adobe Flash 11.x, the sound quality in videos played on websites like fix news and cnn is distorted and scratchy. This does not occur when playing native video or audio files, files within iTunes or when playing the same videos from internet explorer on the same website listed above.
Opaite Mbohovái (1)
There have been some issues with the Flash player 11.3 plugin. Adobe listed "audio distortion issues when streaming Flash content" as a known problem in its last release notes: http://helpx.adobe.com/flash-player/release-note/enduser-release-notes-11_3.html
Note that IE uses a different binary, the ActiveX player (.ocx) vs. the "Netscape style" plugin (.dll) used in Firefox, so that could explain the difference.