One of those problems that some Xperia Z5 owners have been complaining about is the low in-call volume. This seems to have affected users only after they have updated to Nougat, which suggests that the update could be the main problem. The good news is that Sony has since acknowledged the issue and that they will be issuing a fix soon.
How soon, you ask? Apparently users will still have to wait another month as Sony only plans on releasing the update in April. The update will be rolled out in stages so even then you might not receive it so soon. It will also probably vary country to country and carrier to carrier, but hopefully this is something that users can look forward to without too long of a delay.
Now if you’d rather not wait, there is an alternative as Sony has suggested that a factory reset of the phone should solve the problem. You will of course want to backup your device completely to prevent any loss of data before you proceed.