Why does music keep playing during videos? Why does music play when I'm watching videos?

Sorry re lack of detail.  I have a Mac Air. I use Firefox. 

It first happened maybe a year ago when my son sent a little home video taken on iphone. There was this slow dirge-like repetitive music playing in the background--loudly, too, so that I couldn't hear the voices on the video. When I lower the volume, the background music also decreases, along with sound on the video.  

That continued for several months and happened for videos from the web as well. It then stopped for a couple of months and today it's back.  Watching SNL iconic episodes on Yahoo.

I should add that this same tune plays continuously, whether a video is playing or not. So I keep the volume muted when just using the web.
So, music always plays when you have web browser open, regardless of any videos?

Maybe.. check if there are tabs open, try to close them one after other if you would find if one of those has background music. Chrome and some other browsers might show an icon indicating if a tab is playing music.