Yes, this is all MP scenes, and this has been an outstanding issue at least since September/October. Numerous other people have mentioned it in the modding discord, I'm just the first one to make a bug report about it since it wasn't getting fixed in subsequent patches. It should be trivial to replicate in one of two ways:
1) Make an MP scene in the editor, add an ambient sound effect (or a master sound effect), put the scene on a server, and play on it. You won't hear the sounds.
2) Alternatively, have someone in QA look at the MP scenes which already exist in the editor and search for where ambient sounds have been placed, then go stand in those areas on an MP server and you will see they don't play. Also, the scene makers themselves should be able to pretty easily determine that the sounds they put in their scenes aren't working the way they should be.
For example, after verifying integrity of game files just now I hopped on TW official MP servers and saw that Azar Stronghold (mp_siege_map_009) should have 'desert_ambient_sound' playing, but is instead silent.
Let me know if you have any other questions or would like to discuss further.