Yesterday while in a GV battle I heard some others mention they were having a hard time hearing the GVs approaching because the sounds were like they were in a cave and echoing. At that time I didn't experience it.
But, low and behold my next GV battle I had the exact same thing happen. That's the first time I've heard that, ever.
Simply landing and re-upping seemed to cure the problem.
As an added note, my problem with being able to hear GVs 3.5k (planes 5.2k but not at full volume) out at near full volume still exists. The volume does increase as they get closer, but just a small amount.