This tutorial will go over the super-simple method for creating a song Easter Egg for your custom map, and will be the first of many new 'n' fresh tutorials, such as custom weapons and game-altering elements.
Before we begin, be sure you have the latest FGD, which you can get via this link (right click, save as): NZ:P Reboot FGD
After you're finished and have linked that to Jackhammer, you can now create a trigger_song entity through the entity tool.
In the properties tab for this entity, you're able to change the model, skin, and trigger sound. I chose to use the radio model and the BAR's maghit sound for mine.
You can place as many of these entities are you wish, ranging models and activation sounds.
When you're finished, go to the Map > Properties window and point the map to the desired song you want to play when all entities have been activated.
And that's it! Compile your map and try it out.
If you have any issues with this, feel free to leave a comment below or @ me in the Discord server.