This first: 1. Open WMP9. Click Tools -> Options -> Devices -> Speakers -> Properties. Make a note of what's selected in the 'Audio device to use' box. 2. Now select the correct Audio device to use for your PC (depending on your sound card & drivers installed.) Click Apply, OK. Close WMP9. 3. Open WMP9 & try to play a MP3. 4. If it doesn't work you may want repeat the above process & select something different each time. 5. On exhausting all options return the settings to the original (from the note you made in step 1.)
I've checked a range of my MP3 files and found that for some I get a message that the input media format is invalid. For some others I get the player screen and visuals but no sound while the rest play perfectly.