Mute devices when they're being used if the --mute-master option is given.

lib/units/device/plugins/mute.js

@@ -0,0 +1,30 @@
+var Promise = require('bluebird')
+var syrup = require('stf-syrup')
+
+var logger = require('../../../util/logger')
+
+module.exports = syrup.serial()
+  .dependency(require('./group'))
+  .dependency(require('./service'))
+  .define(function(options, group, service) {
+    var log = logger.createLogger('device:plugins:mute')
+
+    if (options.muteMaster) {
+      log.info('Will mute master volume during use')
+
+      group.on('join', function() {
+        log.info('Muting master volume');
+        service.setMasterMute(true)
+      })
+
+      group.on('leave', function() {
+        log.info('Unmuting master volume');
+        service.setMasterMute(false)
+      })
+    }
+    else {
+      log.info('Will not mute master volume during use')
+    }
+
+    return Promise.resolve()
+  })

lib/units/device/plugins/service.js

@@ -671,6 +671,20 @@ module.exports = syrup.serial()
       return Promise.resolve(true)
     }
 
+    plugin.setMasterMute = function(mode) {
+      return runServiceCommand(
+          apk.wire.MessageType.SET_MASTER_MUTE
+        , new apk.wire.SetMasterMuteRequest(mode)
+        )
+        .timeout(10000)
+        .then(function(data) {
+          var response = apk.wire.SetMasterMuteResponse.decode(data)
+          if (!response.success) {
+            throw new Error('Unable to set master mute')
+          }
+        })
+    }
+
     function runServiceCommand(type, cmd) {
       var resolver = Promise.defer()
       var id = Math.floor(Math.random() * 0xFFFFFF)