Add voice guide

docs/guides/samples/audio_create_ffmpeg.cs

@@ -0,0 +1,11 @@
+private Process CreateStream(string path)
+{
+    var ffmpeg = new ProcessStartInfo
+    {
+        FileName = "ffmpeg",
+        Arguments = $"-i {path} -ac 2 -f s16le -ar 48000 pipe:1",
+        UseShellExecute = false,
+        RedirectStandardOutput = true,
+    };
+    return Process.Start(ffmpeg);
+}