Android - How can I change media volume when no media is being played?
You can go to
Settings > Sound > Volumeand turn down the Media volume all the way to 0.I used AutomateIt to Mute Media sound unless the earphone is plugged.
A hack that I used to do is to just cover up the speaker hole until I can turn down the volume.
On AOSP Android 6.0 Marshmallow, this feature is integrated into the OS.
From an article on Pocket-lint,
Android 6.0 Marshmallow: Volume controls
One of the places where Lollipop went a little haywire was with the volume and controlling those new notifications. These new fangled volume controls sent ripples across Android devices on Lollipop with manufacturer skins that followed, and have been tweaked by all and sundry ever since.
On Android 6.0 Marshmallow, they have changed. Hooray!
Hitting volume down or up will open the volume controller and a drop-down arrow will let you change the ringer volume, alarm volume and media volume independently. That means you can ensure that game you're about to start up in bed doesn't blare out music as soon as it opens.
Image courtesy of TrustedReviews
If you have Tasker, you can use it to adjust your media volume based on proximity to a WiFi network and many other conditions. I have mine configured to set the media volume to 1 when in range of the office WiFi (as well as setting the ringer volume to an appropriate level, etc.). You can also configure gestures as triggers, but I have not tried this.