This is happening because your event is looping but the sound module is not set to be asynchronous. Even though this is behaving as expected in the editor (randomly start at different places when played), it does not work as you expect it to in Unity.
Please set your single sound module to be Async (button above the waveform preview in the Deck). Please note that you will need to adjust your loop region to be ever-so-slightly longer than the sound module in order to loop correctly. The playback cursor needs to exit the async sound module and re-enter it in order to trigger again.
You will also need to set the Polyphony limit to 1 in order to not cause several ambiances playing at once, an AHDSR on the volume of the sound module so they fade in and out smoothly when the loop region ends, and set the sound module to loop (top right of the waveform preview in the deck) so that if the offset causes the playback to finish before the loop region ends the sound module loops.
Also, in terms of keeping your ambiences “even”, you can place all ambiences into a group bus and use a compressor and/or a limiter to ensure they do not go over a certain volume.
In regards to your request for a custom shape for the ambiences to be applied, we only support spherical emitters for attenuation currently. We do have a feature requests in our tracker for a future release, but we cannot give a time frame on it.