Don't allow user to unmute if another user is speaking

Based on #2312
For element-hq/element-call#298

Author: dbkr

src/webrtc/groupCall.ts

@@ -59,6 +59,12 @@ export class GroupCallError extends Error {
     }
 }
 
+export class OtherUserSpeakingError extends Error {
+    constructor() {
+        super("Cannot unmute: another user is speaking");
+    }
+}
+
 export interface IGroupCallDataChannelOptions {
     ordered: boolean;
     maxPacketLifeTime: number;
@@ -408,6 +414,12 @@ export class GroupCall extends EventEmitter {
 
         // set a timer for the maximum transmit time on PTT calls
         if (this.isPtt) {
+            // if anoher user is currently unmuted, we can't unmute
+            if (!muted && this.userMediaFeeds.some(f => !f.isAudioMuted())) {
+                throw new OtherUserSpeakingError();
+            }
+
+            // Set or clear the max transmit timer
             if (!muted && this.isMicrophoneMuted()) {
                 this.transmitTimer = setTimeout(() => {
                     this.setMicrophoneMuted(true);