Some people who use Nexus 6P find that their phone speaker is not really has the function as it normally does. It is quite a unique case because it is quite rare. They find that their Nexus 6P top speaker is louder than the bottom speaker.
This problem is quite irritating because it disturb the harmony of the sound when the phone is used to see a movie on landscape mode. Although most of the problem is not really bold, while the difference of the sound level is approximately 40 and 60 split, there still some that has quite a big distance of the sound level between the top and bottom speaker.
You can try to overcome this problem by using the factory reset option available in your phone. This option is commonly offered although the problem is likely not caused by the software problem.
You can do this factory reset by opening the Settings then select Backup and Reset, and tap Factory Data Reset then select Reset Phone and finally please Erase Everything and then reboot.
Please do not forget to manually copy and move the content after you reset it or back up your data before you do this reset because this will erase all of the files in your phone.
Some of the users report that they already tried this factory reset but their problem still remains the same. However, it is quite worthy option to try while you are seeing for other option to overcome your problem.
If your problem is also still remains after you try to use the factory reset, you need to ask any help to Google support.
Because your problem has a big chance as the hardware problem, you need to ask unit replacement to them. You can get the unit replacement than you will asked to send back your problematic unit within one week after you receive the replacement unit.