Aside from the fact that what you're suggesting is overall of very dubious legality, you do realise that if any of us can access a theoretical dark web download site, TLG could do just the same? And why would we want to expose the primary demography for these kinds of games, younger kids, to a potentially dangerous section of the internet?
DOOM was originally shareware, and since its 1993 release, has had a number of open-source releases from the developers. That said, id/ZeniMax could easily enforce their legal entitlements at any time; they choose not to because it's of no consequence to them right now, but that doesn't mean they blanket won't ever.
The law is the law, and we are looking to keep our relations with TLG as polished as possible. This kind of action could potentially cause all kinds of dramas.
https://en.wikipedia.org/wiki/Track_(optical_disc)
Optical Discs, including CD, DVD and Blu-Ray discs, can be burned with multiple tracks. This makes sense; you have different songs (called "tracks") on a normal audio CD. Now, there are different types of tracks, including Audio, Video, and Data. For the LegoRR CD, the developers and/or the authoring company (the publishers responsible for producing and printing the final CDs) put most of the game's content into a Data track, but then put all of the music into audio tracks. This allows the music to be listened to in a stereo player, as well as be usable in-game. Theoretically, for a copy without any CD requirements, you could burn your own audio CD, and have that music play while you're playing the game.
Someone else with more experience with the mechanics and bugs of LegoRR will need to pitch in with explanations for everything else.