fixes #101 - FFMpeg iTunes m4a transcode issue

MediaBrowser.Api/Playback/BaseStreamingService.cs

@@ -769,26 +769,31 @@ namespace MediaBrowser.Api.Playback
         /// <returns>System.Nullable{System.Int32}.</returns>
         private int? GetNumAudioChannelsParam(StreamRequest request, MediaStream audioStream, string outputAudioCodec)
         {
-            if (audioStream != null)
-            {
-                var codec = outputAudioCodec ?? string.Empty;
+            var inputChannels = audioStream == null
+                ? null
+                : audioStream.Channels;
 
-                if (audioStream.Channels > 2 && codec.IndexOf("wma", StringComparison.OrdinalIgnoreCase) != -1)
-                {
-                    // wmav2 currently only supports two channel output
-                    return 2;
-                }
+            var codec = outputAudioCodec ?? string.Empty;
+
+            if (codec.IndexOf("wma", StringComparison.OrdinalIgnoreCase) != -1)
+            {
+                // wmav2 currently only supports two channel output
+                return Math.Min(2, inputChannels ?? 2);
             }
 
             if (request.MaxAudioChannels.HasValue)
             {
-                if (audioStream != null && audioStream.Channels.HasValue)
+                if (inputChannels.HasValue)
                 {
-                    return Math.Min(request.MaxAudioChannels.Value, audioStream.Channels.Value);
+                    return Math.Min(request.MaxAudioChannels.Value, inputChannels.Value);
                 }
 
+                var channelLimit = codec.IndexOf("mp3", StringComparison.OrdinalIgnoreCase) != -1
+                    ? 2
+                    : 5;
+
                 // If we don't have any media info then limit it to 5 to prevent encoding errors due to asking for too many channels
-                return Math.Min(request.MaxAudioChannels.Value, 5);
+                return Math.Min(request.MaxAudioChannels.Value, channelLimit);
             }
 
             return request.AudioChannels;