Add marker interface and move documentation

osu.Framework/Audio/IHasAmplitudes.cs

···
+// Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Audio.Track;
+
+namespace osu.Framework.Audio
+{
+    public interface IHasAmplitudes
+    {
+        /// <summary>
+        /// Current amplitude of stereo channels where 1 is full volume and 0 is silent.
+        /// LeftChannel and RightChannel represent the maximum current amplitude of all of the left and right channels respectively.
+        /// The most recent values are returned. Synchronisation between channels should not be expected.
+        /// </summary>
+        ChannelAmplitudes CurrentAmplitudes { get; }
+    }
+}

osu.Framework/Audio/Sample/ISampleChannel.cs

···
     /// <summary>
     /// A channel playing back an audio sample.
     /// </summary>
-    public interface ISampleChannel
+    public interface ISampleChannel : IHasAmplitudes
     {
         /// <summary>
         /// Start playback.

osu.Framework/Audio/Sample/SampleChannel.cs

···
 
         public override bool IsAlive => base.IsAlive && !Played;
 
-        /// <summary>
-        /// Current amplitude of stereo channels where 1 is full volume and 0 is silent.
-        /// LeftChannel and RightChannel represent the maximum current amplitude of all of the left and right channels respectively.
-        /// The most recent values are returned. Synchronisation between channels should not be expected.
-        /// </summary>
         public virtual ChannelAmplitudes CurrentAmplitudes { get; } = new ChannelAmplitudes();
     }
 }

osu.Framework/Audio/Track/ITrack.cs

···
     /// <summary>
     /// An audio track.
     /// </summary>
-    public interface ITrack
+    public interface ITrack : IHasAmplitudes
     {
         /// <summary>
         /// States if this track should repeat.

osu.Framework/Audio/Track/Track.cs

···
 
         public override bool HasCompleted => IsLoaded && !IsRunning && CurrentTime >= Length;
 
-        /// <summary>
-        /// Current amplitude of stereo channels where 1 is full volume and 0 is silent.
-        /// LeftChannel and RightChannel represent the maximum current amplitude of all of the left and right channels respectively.
-        /// The most recent values are returned. Synchronisation between channels should not be expected.
-        /// </summary>
         public virtual ChannelAmplitudes CurrentAmplitudes { get; } = new ChannelAmplitudes();
 
         protected override void UpdateState()

osu.Framework/Graphics/Audio/DrawableSample.cs

···
 // See the LICENCE file in the repository root for full licence text.
 
 using osu.Framework.Audio.Sample;
+using osu.Framework.Audio.Track;
 
 namespace osu.Framework.Graphics.Audio
 {
···
             get => channel.Looping;
             set => channel.Looping = value;
         }
+
+        public ChannelAmplitudes CurrentAmplitudes => channel.CurrentAmplitudes;
     }
 }

osu.Framework/Graphics/Audio/DrawableTrack.cs

···
         public void Start() => track.Start();
 
         public void Stop() => track.Stop();
+
+        public ChannelAmplitudes CurrentAmplitudes => track.CurrentAmplitudes;
     }
 }