I am curious whether this has ever been done before.

What I am looking for is:
1. A computer or video game
2. whose soundtrack contains aleatoric music (that is, music that contains random elements)
and preferably
3. where the music is generated in real-time (as opposed to just being a recording)

The reason for criterion 3 is that, if you just have a recording, it will sound the same every time; by generating the music in real-time, it can be made to sound different based off what the RNG decides (for example).

Maybe throw some real dice?
https://en.wikipedia.org/wiki/Musikalisches_W%C3%BCrfelspiel

I'm not sure if this applies, but I know that Mark of the Ninja's OST wasn't really composed in the traditional sense, rather simple sounds were recorded that get shuffled and put together during gameplay dynamically, following a defined pattern.

Mark of the Ninja on GOG:
https://www.gog.com/game/mark_of_the_ninja

Does random elements refer to random sound effects? Do those games count that change their music dynamically, add percussion if a scene gets more intense, etc. ?

Have you any example which could make clear what you're after?
Interesting question. I'll think more about it, later.

Edit 2 minutes later:
Monkey Island 2 did something similar with the iMuse system, randomizing instruments and arrangements of existing tracks when Guybrush switched to another screen.

Monkey Island 2 on GOG:
https://www.gog.com/game/monkey_island_2_special_edition_lechucks_revenge

The background music of Spore is afaik randomly generated.

You mean like modern jazz?

NiGHTS into Dreams... uses AI-generated (maybe not so random) music.

The only thing I can think of is Hypotenuse, a downloadable indie game on the 360.
It had an extremely minimalist soundtrack, which I'm fairly certain was procedurally generated.

To be fair, it's basically just individual notes on some string instruments with relatively long time intervals between them.

I think Proteus does this, with a really interesting result.

Random? https://www.youtube.com/watch?v=sC0cvwnG0Ik ;)

Are you really still wondering, or caring, about that sort of thing? To some people pressing that down-vote button is a hobby. *shrug*

On topic, if I remember correctly, No Man's Sky uses procedural BGM to some extent, going with the general game concept.

50 cent said that good people die young - is that why you nasty woman? you want to live long?

Not exactly randomly generated, but Spheres of Chaos springs to mind.

Would this count? Probably not quite random, but procedurally generated, in part, and reacting to the player:
Chompston

Crypt of the NecroDancer use some tricks to make a 10-hour-long vocal music:
https://www.youtube.com/watch?v=I0xzDS7rcuc

In fact, the music could be played even longer...