I am also experiencing a hissing while on calls. I have to lower the volume severely for me to not notice it. If I have audio around 50-60% the voice from a call sounds overblown and hisses. This also happens to me when I use Cortana. If I long press to use voice assistant Cortana sounds horrible, as if the sound was turned way up and her voice sounds distorted and overblown.
I reported this issue months ago but all I got was suggestions on how to factory reset the device and I did but it didn't fix anything. I noticed today there was a new update 3.0.0.6 but when I go to see what's new the Microsoft page it takes me to doesn't even show the latest update, it shows the previous one. Also, in the product page they lowered the amount of hours you can listen to on a single charge from 8 to 7 when using the latest update!