A BMW M135i owner has discovered the cabin engine sound is artificially enhanced, just like in the M5 (F10).
He managed to unplug the sound symposer device and noticed the radio wasn't working anymore. While the car exhaust sounded normal, the low-bass sound of the 6-cylinder engine was gone leaving only a "faint choked turbo six with the dump valve being the loudest thing." He added the differences are more noticeable from 2,500 rpm.
After plugging the device back in, the audio system started working normally and the beefier engine sound was once again available.
Thanks to Declan Lewis for the tip!