What is audio stream?

Android defines several audio streams, each used for different purpose, e.g.:

Why is this important?

Silent Mode Filter can use either alarm or music audio stream. When you mute your phone, Android shuts down the stream for telephone ringer and, depending on the operating system version and device manufacturer, it may shut down alarm and/or music. If SMF does not work, try switching audio streams. If Android shuts down both of them, SMF will unfortunately be unusable.

If you choose music audio stream don't forget to unplug your headphones when you are not listening to music.