Hiya...
You probably have a problem with CODECs - Either they have become corrupted or perhaps deleted in error.
There are several CODEC Packs available... I recommend
>
Cole2k Codecs
Just click on the above link and download the STANDARD pack.
When you have the install file, double-click on it and the simple install program will begin...
Choose the EASY install, NOT the Advanced !
You don't need the offered toolbar.
When installation is complete, Shutdown your computer.
Start again and you should have no problems with any audio or video files.
There is nothing that you need to do after the install and the Shutdown and Startup process. All is automatic and you never need to 'select' anything.
Hope it helps... Let us know, OK ?