diff --git a/GameRes/Audio.cs b/GameRes/Audio.cs
index c1b1e8a5..17c5bbbb 100644
--- a/GameRes/Audio.cs
+++ b/GameRes/Audio.cs
@@ -35,6 +35,7 @@ namespace GameRes
         public   uint AverageBytesPerSecond;
         public ushort BlockAlign;
         public ushort BitsPerSample;
+        public ushort ExtraSize;
     }
 
     public abstract class SoundInput : Stream
@@ -122,6 +123,11 @@ namespace GameRes
 
         public abstract SoundInput TryOpen (Stream file);
 
+        public virtual void Write (SoundInput source, Stream output)
+        {
+            throw new System.NotImplementedException ("AudioFormat.Write not implemenented");
+        }
+
         public static SoundInput Read (Stream file)
         {
             var input = new MemoryStream();