Android - How to disable sound from an app

There are several possibilities:

• manually setting all volumes to 0, except for those channels you want to keep (e.g. alerts, notifications, system). Issue: on some devices/systems some channels seem to be "locked together", so as soon as e.g. volume for calls is increased, some other slider increases automatically along. An app like Audio Control might help on that.
• same thing, but use some automation tool (e.g. tasker). This might work-around the slider-lock.
• disabling notifications from the app (might not work if the app doesn't count that as "notification")
• if root access is available: replace/remove the corresponding sound file (credits go to Dan for this)
• using a helper like App Override. Unfortunately this is no longer available on Google Play, so I linked to the original thread on XDA.

If you happen to be on a Samsung phone, there is an app called "Sound Assistant" which is excellent - adding volume sliders for whatever individual apps you like.

They waste a lot of money and frustrate a lot of people customizing & bloating Android, but this optional app is great and should be mimicked by Google into base Android!