It is not restricted to only live mode. You’ll be able to hear the sounds in solo as well.
The city sounds can be heard when your engine is off and when you’re sitting on the ramp. You may have to turn up the volume if nothing can be heard. The sounds will resemble that of cars and trucks driving on a nearby highway.
The other scenery sound you will hear will be a result from wind. If you’re on the ramp and say the winds are 25G40kts, you will notice that a “whooshing” sound will be louder than that of winds calm. You can verify this when you’re flying along as well. If you’re in cruise, you have a tailwind of say 100kts, and you throw the camera view into Free Cam you’ll at first hear and see the plane flying away, until the sound of the wind begins to take over. Its really neat stuff.