Note: This story contains minor, non-story spoilers for Control. If you're looking to find everything yourself, you might want to stop reading now.
Check just about any corner, nook, or cranny in Control and there's something interesting to find. There are tons of clues about the game's story and its world in the form of memos and reports from the Federal Bureau of Control that hint at all sorts of supernatural situations the FBC has encountered. A few details call back past Remedy games like Max Payne and Quantum Break. And there's a lot of material that links Control to Remedy's 2010 game Alan Wake. But one of the coolest and most out-of-the-way Easter eggs has nothing to do with any of that.
In addition to being filled with references to its own games, Control also has a few fun musical Easter eggs. The Finnish studio has a longtime relationship with rock band and countrymen Poets of the Fall, whose music appears in Control, Alan Wake, and Max Payne 2. Control sneaks in another Easter egg related to a Finnish band: Socks and Ballerinas, posters about whom you might have seen scattered around a few places in The Oldest House.
The bigger Socks and Ballerinas Easter egg is located at the top of the big, tall room marked Central Research. In that area, you can find a room where the FBC was testing the "paranatural" effects of the Poets of the Fall song "My Dark Disquiet." Climb and fly to the top of the room to find another lab (in true weird fiction fashion, you can't actually get to this one by any staircase) to discover another sound lab, where a Socks and Ballerinas song is also being studied. Hit a button on a nearby control panel and the song starts playing in the lab and makes objects located within it fly to the ceiling.
As Reddit user Jedi-Outcast discovered, there's a little something more to the room than just an opportunity to listen to the song. On a nearby chalkboard is a message in code, which Jedi-Outcast realized was an A1Z26--one in which a series of numbers correspond to the letters of the alphabet. Filling in the letters revealed the phrase "socksandballerinas." Beside that is listed a chemical compound in molecular form: C17H35COON8. Jedi-Outcast used some chemistry knowledge to recognize it as a soap molecule.
Googling "Socks and Ballerinas Soap," led to the Bandcamp page for the group and its album. Jedi-Outcast writes that they were at a loss as to what the codes at the bottom of the chalkboard referred to, until they discovered a "Redeem Code" link on the sidebar of the page.
Dropping in one of the codes from the chalkboard provided a free copy of the album--a reward for the first three people in the world to solve the puzzle. As a response from Socks and Ballerinas on the post notes, Jedi-Outcast was the second.
All the codes have been redeemed as of this writing, so that part of the puzzle is over. Still, as far as Easter eggs go, this is an interestingly elaborate one and a pretty cool nod to Remedy's attention to music in its games and to Finnish bands in particular. While you can't get it for free, you can still check out the Socks and Ballerinas album on Bandcamp and Soundcloud.