Someone posted this on reddit, ChocoTaco also commenting on how much better sound is. Amusing that this isn't in the patch notes, because this is shit people have really been asking for.
After playing 6 matches on the test server, here are a couple things that stood out to me as major leaps forward in regard to audio design.
Doppler Shift: This phenomena is something we are all familiar with in reality. It's what happens when an emergency vehicle siren appears to change pitch as it approaches and then passes you. This is most noticeable now with the airplane flying overhead, however it seems like a less intense version of this effect is also present with other vehicle sounds.
Massively Improved 3D Audio: Positional audio has been all over the place in the past. Consider the days of frustrating 3D audio over. The sound stage is much larger, and appears to have very fine detail in regard to where sounds are coming from. It didn't occur to me until after my 3rd or 4th match that the panic of not knowing where a sound was originating from was gone. It's very natural sounding now, and likely helped in part by the next point
Reworked Ambient Sound Mix: Not once did I hear any of the background footstep or vehicle noises that plagued the previous mix. In addition to this, the natural sounds such as wind and rustling leaves/fields are much lighter and faint, rather than muffled and intrusive. This helps a ton with the overall quality of the audio mix in general.
Combat Audio: Many of us have probably tried to understand locating shots better at some point during our time playing PUBG. It can be tricky and confusing at first; trying to hear the impact and then the crack of the shot afterwards. Not anymore. Because of the combination of all the previous changes and improvements, there is now a very distinguishable, precise, way to locate the origin of enemy fire. This is something that requires playing the test build to fully grasp it, but you'll hear the difference immediately the first time you get into a distance engagement.