Add sound output to sound card while dumping is occurring. Uses a proxy ISoundProvider (DualSound.cs) to guarantee emulation sound core and dumping routine are unaffected, while sending "best effort" sound to system output. Doesn't sound great, but simple and functional.

BizHawk.Emulation/BizHawk.Emulation.csproj

@@ -295,6 +295,7 @@
     <Compile Include="Properties\svnrev.cs" />
     <Compile Include="QuickCollections.cs" />
     <Compile Include="Sound\CDAudio.cs" />
+    <Compile Include="Sound\Utilities\DualSound.cs" />
     <Compile Include="Sound\Utilities\Equalizer.cs" />
     <Compile Include="Sound\YM2612.IO.cs" />
     <Compile Include="Sound\YM2612.Channel.cs" />

BizHawk.Emulation/Sound/Utilities/DualSound.cs

@@ -0,0 +1,109 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace BizHawk.Emulation.Sound.Utilities
+{
+    /// <summary>
+    /// provides pass-through sound for the dumping tool to use, while making a "best effort"
+    /// to have something available for audio output
+    /// </summary>
+    public class DualSound : ISoundProvider
+    {
+        /// <summary>
+        /// implementation of a "slave" ISoundProvider that recieves best effort audio
+        /// </summary>
+        class SecondPin : ISoundProvider
+        {
+            /// <summary>
+            /// the source to draw from
+            /// </summary>
+            DualSound master;
+
+            public SecondPin(DualSound master)
+            {
+                this.master = master;
+            }
+
+            public void GetSamples(short[] samples)
+            {
+                int i;
+                for (i = 0; i < Math.Min(samples.Length, master.ringbuffer.Count); i++)
+                    samples[i] = master.ringbuffer.Dequeue();
+                for (; i < samples.Length; i++)
+                    // underflow
+                    samples[i] = 0;
+            }
+
+            public void DiscardSamples()
+            {
+                master.ringbuffer.Clear();
+            }
+
+            public int MaxVolume
+            {
+                // ignored
+                get;
+                set;
+            }
+        }
+
+        /// <summary>
+        /// original input source
+        /// </summary>
+        ISoundProvider input;
+
+        /// <summary>
+        /// threshold at which to discard samples
+        /// </summary>
+        int killsize;
+
+        /// <summary>
+        /// storage of samples waiting to go to second pin
+        /// </summary>
+        Queue<short> ringbuffer;
+
+        /// <summary>
+        /// get the slave pin
+        /// </summary>
+        public ISoundProvider secondpin
+        {
+            get;
+            private set;
+        }
+
+        /// <summary>
+        /// default constructor
+        /// </summary>
+        /// <param name="input">the ISoundProvider to use as input</param>
+        /// <param name="buffsize">how many sample pairs to save for the second pin</param>
+        public DualSound(ISoundProvider input, int buffsize)
+        {
+            this.input = input;
+            killsize = buffsize * 2;
+            ringbuffer = new Queue<short>(killsize);
+            secondpin = new SecondPin(this);
+        }
+
+        public void GetSamples(short[] samples)
+        {
+            input.GetSamples(samples);
+            if (ringbuffer.Count >= killsize)
+                ringbuffer.Clear();
+            foreach (var sample in samples)
+                ringbuffer.Enqueue(sample);
+        }
+
+        public void DiscardSamples()
+        {
+            throw new Exception("Dumpers should never discard samples!");
+        }
+
+        public int MaxVolume
+        {
+            get { return input.MaxVolume; }
+            set { input.MaxVolume = value; }
+        }
+    }
+}

BizHawk.MultiClient/MainForm.cs

@@ -33,6 +33,11 @@ namespace BizHawk.MultiClient
 
 		//avi/wav state
 		IVideoWriter CurrAviWriter = null;
+        /// <summary>
+        /// an audio proxy used for dumping
+        /// </summary>
+        Emulation.Sound.Utilities.DualSound DumpProxy = null;
+
 
 		//runloop control
 		bool exit;
@@ -1874,8 +1879,9 @@ namespace BizHawk.MultiClient
 					//TODO - this will stray over time! have AviWriter keep an accumulation!
 					int samples = (int)(44100 / Global.Emulator.CoreOutputComm.VsyncRate);
 					short[] temp = new short[samples * 2];
-					Global.Emulator.SoundProvider.GetSamples(temp);
-					genSound = false;
+					//Global.Emulator.SoundProvider.GetSamples(temp);
+                    DumpProxy.GetSamples(temp);
+                    //genSound = false;
 
 					CurrAviWriter.AddFrame(Global.Emulator.VideoProvider);
 					CurrAviWriter.AddSamples(temp);
@@ -1891,10 +1897,16 @@ namespace BizHawk.MultiClient
 				PressFrameAdvance = false;
 			}
 
-			if (genSound)
-				Global.Sound.UpdateSound(Global.Emulator.SoundProvider);
-			else
-				Global.Sound.UpdateSound(NullSound.SilenceProvider);
+            if (genSound)
+            {
+                // change audio path if dumping is occuring
+                if (DumpProxy != null)
+                    Global.Sound.UpdateSound(DumpProxy.secondpin);
+                else
+                    Global.Sound.UpdateSound(Global.Emulator.SoundProvider);
+            }
+            else
+                Global.Sound.UpdateSound(NullSound.SilenceProvider);
 		}
 
 		/// <summary>
@@ -2704,16 +2716,23 @@ namespace BizHawk.MultiClient
 				aw.Dispose();
 				throw;
 			}
+            // buffersize here is entirely guess
+            DumpProxy = new Emulation.Sound.Utilities.DualSound(Global.Emulator.SoundProvider, 8192);
 		}
 
 		public void StopAVI()
 		{
-			if (CurrAviWriter == null) return;
+            if (CurrAviWriter == null)
+            {
+                DumpProxy = null;
+                return;
+            }
 			CurrAviWriter.CloseFile();
 			CurrAviWriter = null;
 			Global.OSD.AddMessage("AVI capture stopped");
 			AVIStatusLabel.Image = BizHawk.MultiClient.Properties.Resources.Blank;
 			AVIStatusLabel.ToolTipText = "";
+            DumpProxy = null; // return to normal sound output
 		}
 
 		private void SwapBackupSavestate(string path)