feat(IFrameApi): setAudioOnly command & event.

modules/API/API.js

     requestEnableVideoModeration
 } from '../../react/features/av-moderation/actions';
 import { isEnabledFromState } from '../../react/features/av-moderation/functions';
+import { setAudioOnly } from '../../react/features/base/audio-only/actions';
 import {
     endConference,
     sendTones,
             sendAnalytics(createApiEvent('set.video.quality'));
             APP.store.dispatch(setVideoQuality(frameHeight));
         },
+        'set-audio-only': enable => {
+            sendAnalytics(createApiEvent('set.audio.only'));
+            APP.store.dispatch(setAudioOnly(enable));
+        },
         'start-share-video': url => {
             sendAnalytics(createApiEvent('share.video.start'));
             const id = extractYoutubeIdOrURL(url);
         });
     }
 
+    /**
+     * Notify the external application (if API is enabled) when the audio only enabled status changed.
+     *
+     * @param {boolean} enabled - Whether the audio only is enabled or not.
+     * @returns {void}
+     */
+    notifyAudioOnlyChanged(enabled) {
+        this._sendEvent({
+            name: 'audio-only-changed',
+            enabled
+        });
+    }
+
     /**
      * Disposes the allocated resources.
      *

modules/API/external/external_api.js

     sendEndpointTextMessage: 'send-endpoint-text-message',
     sendParticipantToRoom: 'send-participant-to-room',
     sendTones: 'send-tones',
+    setAudioOnly: 'set-audio-only',
     setAssumedBandwidthBps: 'set-assumed-bandwidth-bps',
     setBlurredBackground: 'set-blurred-background',
     setFollowMe: 'set-follow-me',
     'avatar-changed': 'avatarChanged',
     'audio-availability-changed': 'audioAvailabilityChanged',
     'audio-mute-status-changed': 'audioMuteStatusChanged',
+    'audio-only-changed': 'audioOnlyChanged',
     'audio-or-video-sharing-toggled': 'audioOrVideoSharingToggled',
     'breakout-rooms-updated': 'breakoutRoomsUpdated',
     'browser-support': 'browserSupport',

react/features/external-api/subscriber.ts

         APP.API.notifyOnStageParticipantChanged(participantId);
     }
 );
+
+/**
+ * Updates the on audio only value.
+ */
+StateListenerRegistry.register(
+    /* selector */ state => state['features/base/audio-only'].enabled,
+    /* listener */ enabled => {
+        APP.API.notifyAudioOnlyChanged(enabled);
+    }
+);