PlayTime is a free portable tool which calculates the total play time of any mix of audio and video files.
It's straightforward to use. Add files, or folders, or drag and drop your target files from Explorer, and the total time is immediately displayed.
A separate "Selected time" box displays the length of whatever files you've selected, and PlayTime calculates the total file size, too.
The list works just as you'd expect. You're able to drag items around to reorder them, double-click something to play it, add other files or folders as required, or select files and press Del to remove them.
Once you're happy, the list can be exported as a CSV file or a playlist (an M3U).
It's probably easier to total up the playing time of your media files in PlayTime than Explorer, but only marginally. Still, it might be useful if you can use the playlist or CSV export.