@zelphirkaltstahl I replied on the stream, but not sure if it was recorded :)
It's overlay created by eros. It's enabled in Arei by default (maybe later we will re-implement it in a slightly different way): https://git.sr.ht/~abcdw/emacs-arei/
I also packaged geiser-eros in Guix in case you use geiser.
@abcdw I was only able to view it visually, since I had no sound due to things on my end, but I will check out the whole stream later! Thank you.