Try playing some loud music on the phone for an hour.
What version of Android were you running before? Did you update OTA? If so try manually flashing the update. I find updating OTA always introduces bugs.
If nothing above works, google for a thread on XDA titled "[MOD] Headphone & Speaker Volume Boost v3" and install that. You need to be rooted though to use it.
I found not all Xperia Z5 devices have the same speakers, some are not as loud as others, and there are also different revisions of the Z5. If you still have the original box check if your Z5 is Rev,1,2,3,4, or 5. Some are made in China, some Malaysia
