I wanted to ask if it is possible to have the subtitles background in movies semi-opaque... I've tried to have
use_d3d_display
d3d_disp_2d_overlay_alpha 0.6
and movsubtitles_bg_color 0 0 0 128
but it's still solid black.

I did. What do you mean by that?

those are the last few lines:
A couple of the above config vars have separate counterparts for subtitles during movies/cutscenes. They work just
as above except that they are used instead of the above config vars during movie playback.

movsubtitles_bg_color

movsubtitles_bg_textwidth

movsubtitles_dim
so the last line relates to
subtitles_dim <left> <right> <bottom>
    Placement and size of subtitle display rectangle, specified as percentages of the screen. Default is 20 80 93.
    Subtitles are bottom aligned, so the first subtitle will start at <bottom> and go upward. How far upward
    depends on "subtitles_max_count" and how many lines of text each subtitle requires.

do I have the right version of subtitles.txt file?

Have you ever wondered why people leave forums when someone fails to communicate with them like an adult?