Specifically, Xperia Z5 users claim their smartphones have issues with the in-call volume after updating to Android 7.0 Nougat. Many have already taken it to Sony's official support forums to complain they experience low in-call volume issues on their Xperia Z5 phones.
Sony responded after about a month and said that Xperia Z5 users would have to wait another month until the bug will be fixed. A Sony official confirmed that the company has acknowledged the problem and will deliver a software update that should address the “low in-call volume” issue.
The update is expected to hit Xperia Z5 units sometime in mid-April, but since it will be rolled out in stages, timings and availability may vary by market and carrier.
In the meantime, the Japanese company offers a workaround for those who are affected by the issue. According to Sony, performing a factory data reset should eliminate the problem. However, make sure that you backup data stored on the internal storage before doing a factory data reset.