It's finally happened. Some enterprising BMW owner has swapped the faux engine noise pumped through the car's stereo and made their M135i sounds like an M4.
Active Sound Design (ASD) has been around since the latest M5 was introduced, taking the throttle inputs and engine revs from the ECU and using them to pump augmented acoustics into the cabin through the car's stereo. It's now a staple of most performance-focused BMWs, including the M3/M4, M135i, and the i8.
We don't have any details on how this M135i owner supposedly pulled off the audio swap and it's hard to call this definitive without a before and after, but it does sound different. Here's a couple onboard videos of the original cars to compare and contrast: