I recently upgraded to Mountain Lion on my MacBook. Yesterday my sound was OK at my favorite coffee shop. When I got home, my computer was stuck on mute. No red light. iTunes would not move. YouTube would work for 11 seconds. Today I returned to the coffee shop and my sound was fine. When I got home, same problem, stuck on mute. I went to System Preferences > Network > Advanced... > Wi-Fi and I deleted the coffee shop network from my list. I restarted my computer, and now everything works fine again.