Turn down app volume in Lollipop while not in app?
The only way I'm aware of to set media volume outside of an app is to go into the Settings. Specifically, Settings → Sound &amp; notification should give you separate sliders for each of the available volume channels. You'll want to turn down the top one ("Media volume").

On some custom ROMs it may be possible to do this slightly differently. Older versions of CyanogenMod had an option to expand the volume slider overlay, so you could hit the volume button and then manually adjust the appropriate slider from the overlay without having to go into your settings. Other ROMs may include something similar, but I cannot state definitively any that do.

As an aside: I'm not sure that this is unique to Lollipop. I believe it's been like this for a while (as long as I can remember, anyway).