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()
+  })