@Eric
I had the same problems and don't like to mess around with .NET errors that will come up with every .NET update.
Here is a workaround that will do the job in either case:
1. Download mkvtoolnix (
www.videohelp.com/tools/MKVtoolnix), tsmuxer (
www.videohelp.com/tools/tsMuxeR) and eac3toGUI (
www.digital-digest.com/software/EAC3toGUI.html) and install the programs into a single directory
2. Open tsmuxer and load your mkv file
3. uncheck everything but the DTS stream and select it
4. use the output option "Demux" and set a filename for the DTS file and "start demuxing"
After the task has finished
5. open eac3toGUI: Under "settings", you have to enter the path to eac3to.exe (only once)
6. Load the DTS file and check the output file name/directory
7. For best results set AC3 bitrate to 640 kbit
After the task has finished
8. Open mkvmerger.exe (from the mkv tools)
9. Load the original mkv file and add the created ac3 file
10. In the second window - select the added ac3 file (should be the last in the stack) and push the up botton until the ac3 file is at second place
11. Change the "Standard Track Flag" in the track options to "YES" (highest priority for playback)
12. Select the language code from the dropdown selection for your ac3 file
13. Select the output name and directory and "START MUXING"
The result: you're getting the original mkv file with an added AC3 stream that plays first on file load.