An unusual discovery I didn't want to leave this undocumented

Hey again folks.
Recently was messing around with the PlayStation 2 magazine demo disc again (the one containing SHAR and Scrooby Doo Mystery Mayhem). Previously I tried swapping the Demo disc and SHARs disc while the game was still running, nothing interesting happened. Did this in vice versa and got the same result: an error message saying please insert the Simpsons Hit and Run PlayStation 2 disc.
Anyway here's where it gets interesting, I had another game on that disc, Scooby Doo Mystery Mayhem. So I decided to boot up my copy of the game, loaded a save at the start of level 2 (The movie themed level) and swapped the official disc for the demo disc. At first I got some strange really high pitched audio but besides that everything seemed normal until I went to switch characters from Scooby to Shaggy. Normally they do a little high five and say a quip. However this time instead of a quip I heard " - to the touch" I immediately recognised the line as the end of one of Homers lines when speaking to Apu in the Kwik - E - Mart. To my amusement this happened every character swap. Eventually more and more shar lines popped up as I played through the level. Scooby squeezed through a tunnel and said one of Abes lines, catching ghosts yielded the generic pedestrian lines, collecting clues would trigger Barts tutorial lines. I didn't hear any other notable character lines outside of Homer, Bart, Moleman and Abe.

I'm curious, does anyone with more technical knowledge than me know why SHAR line's were triggered when swapping my Mystery Mayhem copies disc with the ps2 mag demo disc?

(Side note there's something surreal about Shaggy being hit by a ghost and yelling "Ow my ass" in a a slightly pitch shifted homer voice.

It is technically possible in some cases depending on how the two games used are programmed, there are many exceptions.
I would estimate that the following conditions all need to be met:

• The game's executable/core program must always remain in RAM (this should always be the case anyway).
  
• The game should load one level entirely into RAM at a time along with any other needed assets, such as player/enemy models.
  
• The game should rely on disc-streamed audio for things like music and voice clips (sound effects often get pre-loaded).
  
• The game should not implement safeguards such as checks on exact audio file names (over sectors) or disc presence.
  
• The location of audio data on both discs should be similar, or the games should use the same audio file names if safeguards are present.

In many games this will work. Weird pitch or skipping are to be expected because games often hardcodedly assume or predetermine during crucial loading sessions things such as sample rates, loop points, exact lengths and locations, and so forth. In many other games it will be dectected and break, in more you probably get static or loud noise. It also isn't very useful for games like TSH&R, which don't load whole levels into memory at once and instead dynamically load chunks in/out as you move around.

That makes alot of sense. Thanks Addy. Now I'm curious to see if replacing the non demo disc version of Mystery Mayhem with a non demo disc version of SHAR has the same results.