Thank you very much for all that information and that explains why the ports of Arcade FMV Games from Mega/Sega CD are like that.
You see, the digital audio and video was an old idea... but only until late 70s they could create chips powerful enough to process fast enough, MANY 1s and 0s (digital) to get some GOOD sound, enough for the human ears to be excellent in quality, as if they were real live. Then, they developed the optical CD to put LOOOTS of those 1s and 0s in just one disc, to be able to put around 70-80 minuts of excellent quality sound. That was when the Audio CDs appeared in the market, in the early 80s, which suceded the old analogical vynil discs and tapes. It had better quality, without the "wear and tear" of those old media using physical "readers".
To listen an audio CD, you would need to have one CD player with a DAC chip (DIgital to Analog Converter chip,
and THAT is the result of the technology developed during the late 70s), an a laser to read the info in the disc. The laser would read that data and send the data of the disc to the DAC chip, in form of a continuos stream of bits (1s and 0s). Then, this DAC chip would translate that info (at the same time is continuously entering) and reproduce it in form of wave sounds, using the speakers connected to the player (or maybe to some headphones, basically mini "personal speakers").
As those "sounds" inside the CD are digital, so they can also be considered pure computer data, and that means a CD could also get around 600-700 MB, equivalent to those 70 or 80 minutes of sound (later CDs, even had more space, like 800MB, squeezing a little the physical space between the data).
Then, someone thought they could also put, in those discs, any type of data, not only "data to be translated to sound", and they created the CD-ROMs for the computers. Eventually, also used in consoles.
With the video, as images are more complex than the audio, it took them a lot more time to get chips able to process an excellent image in digital video form, as well as a good frame rate for them. That happened around 2000s.
In the late 80s and early 90s, when Mega-CD was created, its digital chip to reproduce video was very limited. In fact, I believe the chip Mega-CD used for this... was the VDP (Video Display Processor) of the Mega Drive itself, a console from 1988.
So... those Mega-CD videos had to be very mediocre in resolution, colors, and even frames, compared to the original LaserDiscs games (which were pure analog signals) to be processed by that chip. But it was amazing anyways to see a domestic console doing that.
Anyways, those Mega CD games with lots of videos, also had sound in those videos, but generally that sound is "bad". Why that sound is not as good as it could be, using a CD console? because they are compressed sounds to be "glued" to the videos. Mega CD can read audio CD, and can also play digital videos... but cannot do both things at the same time: only has 1 laser (obviously).
So, when some video is playing, it needs to process the video AND the compressed sound it has "glued", to get video plus sound.
That is why those games with lots of videos doesn't sound as good as the music in Sonic CD, while you are playing a "present stage" with its CD music (just an example). That stage of the game is already loaded in the RAM of the console, so the laser is then able to read the soundtrack of the CD in real time. So... that game sounds a lot better, in the same console.
(In Mega-CD, you obviously cannot load a full video in the RAM. There is not enough space there).