Since the last patch (or the one prior?), there's been several occasions where I cannot hear enemy movement whatsoever up until the moment they start firing. Ziplining, gliding in from a balloon, sometimes even just normal running - it's like the 'outer radius' of how far I can audibly detect enemies shrinks considerably from time to time.
I can't identify anything more specific about it, there's simply moments where enemies can't be heard moving around. I think something got tweaked the wrong way the last time you messed with the mix.
p.s. Your bug reporting on the forums is itself bugged, it refused to allow me to submit a slightly more detailed report about this no matter what I tried to do. All the required fields were filled and everything.