Recently I want to create some scenarios that will recreate race challenges from Flash Games such as Burnin' Rubber series. I wonder if I could add soundtrack to create the feel. Also how could I add Automation car mods as a default car in the scenario creation (as well as opponent cars)? For example in West Coast and I want to have a player car and 3 opponent cars (all automation mod cars) to race each other and you must win first, how should I do it? Note it's not like your typical introduction to scenarios since it involve many more different aspects. As well as adding background soundtrack to recreate the atmosphere. I would prefer a mp3 file as the background music for the scenario. I hope the mods can add the background music feature for the scenarios in the future.
For most of this, I would strongly recommend Nadeox's Introduction to scenarios creation tutorial. That will cover how to set up the scenario and at least get the basics going, including vehicle selection. Now, for music, you can just include an SFXemitter object in the scenario's prefab, and set the emitter's 'filename' field to link to the music in your mod folder. Unfortunately, BeamNG only takes WAV files iirc. If done right, the music should play nonstop on a loop.