Mick Gordon's soundtrack for the 2016 Doom reboot is laden with Easter eggs. There's a creepy "Jesus loves you" backwards-tracked in it, and images of a pentagram and the number of the beast visible in spectographic visualizations. It's probably not surprising that he's done it again for the Doom Eternal soundtrack, which as redditor xXxSlavWatchxXx discovered contains an image that old-time Doom fans will find very familiar.
The track is called Welcome Home Great Slayer—you can listen to it here—and the image is very clearly the cover art from Doom 2. Audio engineering wizardry! I kind of hate to spoil the magic, but there are actually multiple tutorials explaining how to do this online—here's one from audio guy Joshua Casper—and by all appearances, and with all due respect to Mick Gordon, it seems like a pretty simple process. (Bear in mind that I say this as someone still fiercely dedicated to Winamp, though, so my audio bona fides are what you might charitably call lacking.)
If you're hesitant to take Reddit at its word, you can see the image for yourself quite easily with the Sonic Visualizer playback software. I tried it, and it works well: My image quality is poorer, but that's based on a quick, low-quality rip of the YouTube recording, so I think that's to be expected. Honestly, I'm just happy enough that it's there at all.
It's a fun little secret that's on the verge of becoming a tradition, and for me it makes the current controversy surrounding the Doom Eternal soundtrack even sadder: When users noticed that the Doom Eternal OST sounded a little off, audio blogger thatACDCguy explained why—the mix was botched, basically—after which Gordon weighed in to say that he was largely uninvolved in the process.