Maybe you could use
THPView? It has an option for extracting the frames + sound from the THP files, but considering that MTH files are just muted THPs with minor header differences, *maybe* you can use it for this purpose. I haven't tested it yet with MTH files.
Uncompress the RAR in the directory where you have the MTH file you want to work with, then run the Command Line utility (cmd) in that folder by using Clickie (way faster than using the 'change directory' command). Finally, use "thpview -x your_video_file.mth" (without the quotes) in the CMD window and see if it works.