DTS-HD is a family of proprietary audio codecs. The most common application is DTS-HD Master Audio - a lossless codec that is used on many blu-ray discs.
July 2015 UPDATE: Starting from version 1.9.3 MakeMKV uses an open source DTS-HD decoding library without the need for any additional files. The information below is outdated and is preserved only for historical reasons.
Starting from version 1.7.5 MakeMKV can decode DTS-HD audio streams and save them in any supported format (LPCM, FLAC, etc). More details about MakeMKV audio transcoding capabilities may be found on our forum. Unfortunately, MakeMKV still needs an external decoder to decode DTS-HD and a one-time setup is required.
Setting up dtsdecoderdll.dll
In order to decode DTS-HD, MakeMKV needs a so-called reference decoder DLL. Please don't be confused - MakeMKV only uses this DLL as "algorithm reference". With this DLL, DTS-HD decoding will work on all platforms - Windows, Mac OS X and Linux, 32-bit and 64-bit. The fact that this DLL is a Windows 32-bit DLL doesn't matter.
You need to obtain a copy of dtsdecoderdll.dll and set it's location in MakeMKV preferences on "Advanced" tab. MakeMKV will load the file and use it to decode DTS-HD audio. There are several versions of this DLL and any version will work equally fine - there is no need to avoid any particular old version or ensure that you have a latest version of the DLL. MakeMKV knows about all versions of this DLL and will refuse to load any unknown or modified version. Even if you obtain this DLL from a questionable source and DLL is infected with a virus or trojan, nothing bad can possibly happen - MakeMKV will just refuse to load the DLL.
There are several ways to get dtsdecoderdll.dll for free. Here we describe how to obtain this DLL technically - you have to check for yourself that a particular action is legal in your jurisdiction. There are many jurisdictions where all methods described below are legal for personal use.
The dtsdecoderdll.dll is part of Arcsoft TMT player software. So, if you are using Windows and have TMT installed, you already have the DLL - just set its path in MakeMKV preferences. If you don't have TMT installed, you can always download a free trial from Arcsoft website (2015 UPDATE: Arcsoft TMT player is discontionued and is not available on Arcsoft website). Once you install the program, the DLL will be in program directory. You don't have to even run the program once. You may as well copy the DLL from TMT directory and uninstall TMT completely. If you use Mac or Linux, you may install TMT on a separate Windows machine, in bootcamp or inside virtual machine, and copy the DLL from there.
If you don't have access to Windows machine, several internet sites offer this DLL as a free download. Just search for "dtsdecoderdll.dll" and you'll certainly find one. As mentioned before, MakeMKV will safely refuse to load any modified version of dtsdecoderdll.dll .
Setting up location of dtsdecoderdll.dll in MakeMKV is a one-time step, and once done will enable DTS-HD decoding on all platforms.