As above if you can find someone with such a cable and get them to bell it out (possibly resistance as well if we think that is going to be a thing) and take some pictures (knowing what the PS1 side of things was would help narrow down uses) that would be nice, though how many of those or similar enough model still exist I do not know.
Most VCD things I saw back when were not external devices (give or take the basic homebrew enabling cheat cartridges but they are functionally just a mod chip for these purposes) though so I am not sure what goes for this, or what particular function either the device serves or the PS1 serves unless it is just a CD player passthrough in this with all the smarts in the box. That said VCD was far more of a thing in Asia (your flag does say Singapore*) so there is that to consider as well, and if it was around the time then standalone CDs might have been rare just as people used PS2s as DVD players a few years later, and PS3 blu ray beyond that.
I don't think it would be any kind of overlay type device (subtitles and such) at that point.
*I saw I think it was an Indonesian PS1 video out the other day at a car boot sale. Had two prongs on the thing which I had never seen. Probably should have picked it up for the shelf of weird things but hey.
The PS1 has two output options
https://psx-spx.consoledev.net/expansionportpio/
https://psx-spx.consoledev.net/serialportsio/
Plus controller, memory card and whatnot.
Serial port is likely to be too slow to do much in the way of CD reads, though at the same time if the PS1 was 2x CD that is maybe 300 kilobytes a second and thus back in the region of it, not to mention VCD bitrates are 1150 kbits (divide by 8 to get bytes) then still under the limit.
Unless it was serial port mod chip as it were and had some kind of controller built into it.
7 pin socket (8 if you include shielding which is just another ground) does hint towards
https://problemkaputt.de/psx-spx.htm#pinoutssiopinouts rather than expansion (many more pins, though as not all pins are used by all things then not flawless, though here as most of the big stuff is 16 pins and 24 pins less likely, scroll up a bit on the link above for a pinout). It is serial so mostly going to be TX (transmit), RX (receive, remember to reverse TX and RX between the devices as what transmits from one is received by the other and vice versa), voltage, ground, clear to send which you can probably determine easily enough by following traces from whatever chip it comes from inside the box (assuming you can find a pin out of that or play with an oscilloscope).
Probably not going to be RS232 (what most think of if you say serial port) but the principles he uses to reverse engineer it and in some related videos of that little series are pretty much identical to what you would want to do in the absence of an existing cable to bell out.
