DOWNLOAD CUSTOM SONG FOR YOUR PLATFORM
INSTALL INSTRUCTIONS:

 

• PC:
  • DOWNLOAD our tool to install the fix
  • Copy file: "_p.psarc" to "dlc";

• Mac:
  • Follow the instruction over there : https://customsforge.com/index.php?/topic/33889-how-to-add-custom-songs-on-mac/
  • Copy file: "_m.psarc" to "~/Library/Application Support/Steam/SteamApps/common/Rocksmith2014/dlc/", 
    you might need to use "Command-Shift-G" to go inside ~/Library since it's hidden by default in the finder
     
• XBox 360: Place files: "_xbox" to "Content>0000000000000000>555308C0>00000002";
   
• PS3: Install files: "_ps3.psarc.edat" to "/dev_hdd0/game/BLUS31182/USRDIR/DLC",
  if the folder DLC does not exist, just create it.

Note: If you are intending on using Custom DLCs on a PS3 or XBox, you MUST jailbreak your console first! 
Hacking or jailbreaking a console is beyond the scope of what we are doing here, and if you are not comfortable with doing it, you may want to simply buy the PC version of the game.
 
 
JUST PLAY IT !
 
All credit to MarineIguana, he created this, I am merely echoing it here.

Ran through the whole guide again this morning to see if I missed anything, but no dice :(

By the way, the game takes really long to load, like up to ten minutes. Is that normal on Linux?

And it hangs at the moment the pick appears in the bottom right corner. Maybe that means it has trouble finding the .DLL?

Fixed it! I just tried using the new DLL and that did work. Now the game is running quite well; the only hiccup is the long loading time. (Also, I have to plugin the cable before starting the game, and it doesn´t seem to remember the DLC, but those are mostly a nuisance because of the long loading time.)

By the way, I followed these instructions (archive), but with Proton 9.0-4. It's basically Nizo's quick and dirty ALSA guide (archive) with some extra elements.
Then I just downloaded the installer and ran it with protontricks 221680 --> "Run an arbitrary executable (.exe/.msi/.msu)" --> Select the file and execute it. It seems like this is all that was necessary.

(I spent a lot of time trying to set up ASIO, but without any luck, so I'm happy that it's working so well now.)

Quick test to check whether the upload is working well, so I'm all set up for the Championship:

Edited Sunday at 11:19 PM3 days by MaltaSoron
Added archive links for posterity

This isn’t something I could help with, but just a quick note to thank you for posting about the solution, as that may help another person!