I feel like that would make the game more "immersion" per say. Just a neat idea I came up with and I wanted to share it! Even though you can use another application to do so, it would be cool just to have music play in your car and then it muffles when the camera is outside of the car. I doubt this would be added but hey, it's worth a shot.
I remember that there was this radio mod. https://www.beamng.com/threads/beam-radio-v0-6-improved-searching.38879/ Of course, you have to add in the audio files yourself, and I think it's simply played through FMOD (Not sure) but it's something atleast. It works via app. I used it in the past, but it's cool. At the same time, they'd either need non-copyright music to put ingame or use the same way (by putting MP3 files into a folder) to play the songs.
I think that this would be the main reason why they wouldn't do it for "preset music", as it costs money to buy licenses to a song collection site (like APM MUSIC if they were to), or they were able to make their own songs, it would take a while most likely. With the MP3 files, that would probably be more likely to be done out of the 2.
Making their own songs wouldn't be worth it. As far as I know, I haven't heard any sort of ingame music. Either way, the main thing that would have to be coded in is the interior sound and exterior sounds. I have a good feeling that's possible in simply a mod.
I could see that happening if its not done officially, but it would be cool to see it official and in the game for all.
