Correcting how we handle the volume state and UI when users leave and rejoin lobbies, to prevent stale values from persisting and to prevent the initial UI state for a user entering a lobby from being incorrect if they were previously in a lobby.

Assets/Prefabs/UI/UserCardPanel.prefab

   m_Name: 
   m_EditorClassIdentifier: 
   m_volumeSliderContainer: {fileID: 6299655602247130973}
-  m_muteButtonContainer: {fileID: 6569968321909521185}
+  m_muteToggleContainer: {fileID: 6569968321909521185}
+  m_volumeSlider: {fileID: 774196503220678520}
+  m_muteToggle: {fileID: 6442186483341508885}
 --- !u!114 &6299655602247130973
 MonoBehaviour:
   m_ObjectHideFlags: 0

Assets/Scripts/UI/LobbyUserVolumeUI.cs

-using System.Collections;
-using System.Collections.Generic;
+using UnityEngine.UI;
 
 namespace LobbyRelaySample.UI
 {
         private UIPanelBase m_volumeSliderContainer;
         [SerializeField]
-        private UIPanelBase m_muteButtonContainer;
+        private UIPanelBase m_muteToggleContainer;
         [SerializeField]
         [Tooltip("This is shown for other players, to mute them.")]
         private GameObject m_muteIcon;
         public bool IsLocalPlayer { private get; set; }
 
-        public void EnableVoice()
+        [SerializeField]
+        private Slider m_volumeSlider;
+        [SerializeField]
+        private Toggle m_muteToggle;
+
+        /// <param name="shouldResetUi">
+        /// When the user is being added, we want the UI to reset to the default values.
+        /// (We don't do this if the user is already in the lobby so that the previous values are retained. E.g. If they're too loud and volume was lowered, keep it lowered on reenable.)
+        /// </param>
+        public void EnableVoice(bool shouldResetUi)
+            if (shouldResetUi)
+            {   m_volumeSlider.SetValueWithoutNotify(vivox.VivoxUserHandler.NormalizedVolumeDefault);
+                m_muteToggle.SetIsOnWithoutNotify(false);
+            }
+
-                m_muteButtonContainer.Show();
+                m_muteToggleContainer.Show();
                 m_muteIcon.SetActive(false);
                 m_micMuteIcon.SetActive(true);
             }
-                m_muteButtonContainer.Show();
+                m_muteToggleContainer.Show();
-        public void DisableVoice()
+        /// <param name="shouldResetUi">
+        /// When the user leaves the lobby (but not if they just lose voice access for some reason, e.g. device disconnect), reset state to the default values.
+        /// (We can't just do this during Enable since it could cause Vivox to have a state conflict during participant add.)
+        /// </param>
+        public void DisableVoice(bool shouldResetUi)
+            if (shouldResetUi)
+            {   m_volumeSlider.value = vivox.VivoxUserHandler.NormalizedVolumeDefault;
+                m_muteToggle.isOn = false;
+            }
+
-            m_muteButtonContainer.Hide(0.4f);
+            m_muteToggleContainer.Hide(0.4f);
-
-        /* TODO : If we can hook in the volume from a user, we can plug it in here.
-              /// <summary>
-              /// Controls the visibility of the volume rings to show activity levels of the voice channel on this user.
-              /// </summary>
-              /// <param name="normalizedVolume"></param>
-              public void OnSoundDetected(float normalizedVolume)
-              {
-                  m_voiceRings.alpha = normalizedVolume;
-              }
-              */
     }
 }

Assets/Scripts/Vivox/VivoxUserHandler.cs

         private string m_vivoxId;
 
         private const int k_volumeMin = -50, k_volumeMax = 20; // From the Vivox docs, the valid range is [-50, 50] but anything above 25 risks being painfully loud.
+        public static float NormalizedVolumeDefault { get { return (0f - k_volumeMin) / (k_volumeMax - k_volumeMin); } }
-            m_lobbyUserVolumeUI.DisableVoice();
+            m_lobbyUserVolumeUI.DisableVoice(true);
         }
 
         public void SetId(string id)
                     {
                         m_vivoxId = participant.Key;
                         m_lobbyUserVolumeUI.IsLocalPlayer = participant.IsSelf;
-                        m_lobbyUserVolumeUI.EnableVoice();
+                        m_lobbyUserVolumeUI.EnableVoice(true);
                         break;
                     }
                 }
             if (isThisUser)
             {   m_vivoxId = keyEventArg.Key; // Since we couldn't construct the Vivox ID earlier, retrieve it here.
                 m_lobbyUserVolumeUI.IsLocalPlayer = participant.IsSelf;
-                m_lobbyUserVolumeUI.EnableVoice();
+                m_lobbyUserVolumeUI.EnableVoice(true);
             }
         }
         private void BeforeParticipantRemoved(object sender, KeyEventArg<string> keyEventArg)
 
             bool isThisUser = username == m_id;
             if (isThisUser)
-            {   m_lobbyUserVolumeUI.DisableVoice();
+            {   m_lobbyUserVolumeUI.DisableVoice(true);
             }
         }
         private void OnParticipantValueUpdated(object sender, ValueEventArg<string, IParticipant> valueEventArg)
                 if (property == "UnavailableCaptureDevice")
                 {
                     if (participant.UnavailableCaptureDevice)
-                    {   m_lobbyUserVolumeUI.DisableVoice();
+                    {   m_lobbyUserVolumeUI.DisableVoice(false);
-                    {   m_lobbyUserVolumeUI.EnableVoice();
+                    {   m_lobbyUserVolumeUI.EnableVoice(false);
                         participant.SetIsMuteForAll(m_vivoxId, false, null);
                     }
                 }
-                        m_lobbyUserVolumeUI.DisableVoice();
+                        m_lobbyUserVolumeUI.DisableVoice(false);
-                        m_lobbyUserVolumeUI.EnableVoice();
+                        m_lobbyUserVolumeUI.EnableVoice(false);
                 }
             }
         }