Eurorack is a standard for audio processing - it is very popular among musicians, particularly for electronic and noise genres. Lots of modular components that use analog "control voltages" as inputs and outputs, being combined to produce sounds - this is done with actual hardware and tons of patch cables (it is an incredibly expensive hobby), though there are also digital simulations of this available, as well as self-contained hardware that uses muxers to achieve the same thing. These signals can also be used to control other devices based on the sounds you're generating - since the signals are coming from the middle of the pipeline that generates the sound, you can make lights or computer-generated visuals that depend on the music you're making - without having to try to extract the information you want from the final audio signal (which is hard, and is why most general purpose music visualization software (milkdrop for winamp, for example) struggle to generate output that feels really connected to the music).
"Jitter" is also the name of a piece of software used in music visualization circles.
Computer generated visuals, as in, pretty video that is generated based on input from the music (again, think milkdrop, electric sheep, etc). My guess would be that he performs live music generated from his eurorack contraption, blasting from a wall of speakers, and will have this video projected onto a large screen in front of a dance floor filled with drug-crazed lunatics - they call it "electronic dance music", it's what "raves" evolved into.
But to do that, he needs to get these control voltages into the computer - that is, he's going to use the microcontroller's analog inputs and then transmit that to the computer over serial to feed it to the program generating the visualizations. Basically just using the arduino as an ADC.
Anyway - this is not a weird thing to do, and like, I question the value of that block of questions when people well-placed to answer this already have that background knowledge.
Back to OP's questions, while I haven't done this sort of thing, I suspect that some careful googling would pick up a lot of information posted by others who have done this. Lots of people do this.
From a technical perspective, your biggest concerns are how many inputs you need to read, and what sampling frequency you need. That will dictate what boards would be appropriate.