Fear not, there is no need to adjust your sound settings, nor are we all imagining things. In a blog post PUBG Dev have revealed the changes made, and also have apologised for not announcing the changes in the patch notes.
"First things first: We should have explained the change in the patch notes. We messed up here, and we hope you’ll accept our apology.
Many of you have called us out about not being thorough enough about documenting changes in the patch notes, and rightfully so. It’s something we’re going to try our best to fix going forward, and you guys should absolutely continue to hold us accountable when we miss something. We can and will do better."
Primarily the changes to the in-game audio are around the gunshots coming from other players. The HRTF (Head Related Transfer Function) plugin, which was implemented into CS:GO back in December 2016 to some uproar, has been in the game, but did not affect gunshots, but was previously only to do with movement, explosions and other player related noises.
The new changes should make it easier to figure out if gunshots are coming from not only your left or your right as previously, but also if they are in front of you, behind you, or if they are above or below you.
The developers also announced that they plan to do further letters explaining any major changes to the game systems, and not leaving the community in the dark in the future, which is a welcome addition.
PUBG Dev Letter- On Recent Sound Design Changes Steam Community