In my Lenovo 520, I have found that hitting Tab kills the audio. This happens also with tilde (~), Caps Lock, and Shift. This happens if I hit the keys with normal force but if I press them very lightly, the problem does not occur.
The "Play" indicator does not stop; the seconds continue ticking away in iTunes as it shows the song playing; changing the volume makes no difference.
These keys have not become some sort of mapped "speakers off" key. My "speakers off" key works fine. Rather this seems like a hardware problem: Those keys turn off the audio with a tiny buzz-click sound.
But, it also has aspects of a software problem.
In iTunes, I can restart the application to make the audio return.. In Windows Media Player, all I need to do is play a different song. For YouTube in Chrome, reloading starts the audio again.
What's going on?