When I opened my GMail just now, I was surprised to hear some music playing. Even more surprised since only GMail was currently opened in my tab and nothing else. I thought maybe it was an email that I was reading about so I closed it but the music still kept playing.
I closed the tab and opened GMail again but still the music persisted. Then I remembered that they just released a new feature called Gmail Priority Inbox where you can separate out your most important messages making it easy for you to read and respond to the messages that matter.
I tried to check in any Google forum I could come across with and it turns out that when you have not yet activated the setting for Priority Inbox, a YouTube video automatically plays. Weird because when I saw the YouTube screen, it was not even playing.
Anyway, the music stopped when I clicked the link of the Priority Inbox so I could set it up.