Many thanks to Namaiki !
Indeed, there is a problem if you activated "mute" when Media Player Classic was playing a Shockwave Flash file (*.swf). In this case, the solution of Namaiki works.
I suppose it's a bug in Media Player Classic ! I tested it and here is the result :
- To reproduce the bug :
Open any *.swf with MPC.
Activate "mute" (press speaker button) or put the volume at 0%.
Close MPC.
Open any video or *.mp3 with MPC.
You can try to deactivate "mute" and increase the volume, the sound will stay cut off.
- To fix the problem :
Open any *.swf with MPC.
Make sure "mute" is deactivated and the volume is higher than 0%.
Close MPC.
Open any video or *.mp3 with MPC.
The sound works again. (If you don't put the volume at 100% at the previous step, you could have to increase the volume for MPC in the Volume Mixer of Windows 7.)