With regards to cables, the type of connector fitted to the MTX board for both the ear and mic sockets are mono 3.5mm types.
Stereo 3.5mm jack plugs are not compatible, as the contacts don’t line up correctly.
So for the MTX end, you really do need a mono 3.5mm jack plug.
If your source device has a 3.5mm stereo socket, I recommend this “3.5mm STEREO Jack Plug to 2 x MONO Jack Plugs SPLITTER Converter Cable Lead 1m” lead
or a similar one. For loading, as long as the file is stereo, or the PC output is configured to output mono sound to both stereo channels, it does not matter which 3.5mm mono plug you use. If however there is audio on only one stereo channel, then you will have to try each plug in turn.
For both modern computers and other audio sources like CD players, etc, the signal level may not be enough, as the levels are limited compared to the levels available from old cassette decks of the 1980s. So you may need an amplifier. The cheap type of powered speakers for use with a PC are normally okay, as long as they have a headphone output.
Some cassette decks and various other older audio equipment, especially from European companies in the past did use DIN connectors. Three pin for mono, five pin for stereo, and seven pin if motor control was available for the cassette motor. All are backwards compatible. That is a five pin DIN plug goes in and works in a seven pin socket.
Speaking of cassette decks, Argos used to still sell a “shoe box” cassette deck. They still may...