How to Fix "Internet Explorer not playing audio or video files"
The reason for the error is because your defaults have changed to a third party program for playing your audio and video files.
1. Click Start, select Default Programs
2. Click Set program access and computer defaults
3. Click Custom, and select Windows Media Player
4. Click OK.
5. Click Set your default programs
6. Select Windows Media Player from the list
7. Click Set this program as default
8. Click OK.
Note: Also configure third-party player so that it does not takeover the media file associations again.