Very quiet audio output under Windows 10 (Late 2016 MacBook Pro)

Try the Boom system-wide volume booster tool. While it's not free, it is cheap: $6.99. See reviews here, here, and here. Below is the description from the product page, and a screenshot:

Boom is a simple volume booster and system-wide equalizer that lets you boost the volume of your Mac and your music files.

Using Boom is simple. You know the ways to change the volume of your Mac. Boom provides the same easy access to boost sound.

From the videos on YouTube, Hulu, your favorite music playing on iTunes, to voice applications like Skype, iChat and your favorite Games, Boom can boost them all. [...]

(emphasis above is mine)

I use Boom on my own MacBook Air and it does a reasonable job of increasing the internal speaker volume beyond what I was getting with the built-in volume controls at maximum. The graphic equalizer is an added bonus, letting me tweak the sound I get from these tiny speakers so it sounds a bit better for music.

Before you do any of the steps mentioned below the line-break, check the bootcamp control panel first!!!

When in Windows, after everything is installed and done, do the following:

1. Click 'Start ' (the windows logo-button in the bottom left corner)
2. Type 'bootcamp'
3. Open the bootcamp control panel. It will ask for administrator privileges, say 'yes' or 'ok' or whatever
4. Configure the necessary settings for your startup disk, remote, keyboard and trackpad - these are the four tabs you should expect to see when the bootcamp control panel opens.

Your keyboard and trackpad should work (almost) perfectly. If not, read the answer that follows.

When you installed Windows the specific bootcamp drivers should have been installed automatically from your Mac OS X 10.6 (Snow Leopard) DVD or 10.7 (Lion) DMG in the process. I would recommend trying to:

1. Remove/Delete the drivers in Windows Device Manager
2. Insert or mount the OSX DVD or DMG
3. Restart your system (booting into Windows).

The correct bootcamp drivers should then be installed automatically from the OSX DVD. If the drivers were not installed, browse for them on the OSX DVD.

If all else fails you can always try re-installing Windows, and from there should be able to determine during the process if you have keyboard functionality.

★ The key to having the correct drivers working in bootcamp is installing them specifically from the OSX DVD or DMG.

The Mac OS X 10.6 and 10.7 Boot Camp Software includes these Mac drivers for Windows:

Apple Bluetooth
Apple Keyboard Support
Apple Remote Driver
Apple Trackpad
Atheros 802.11 Wireless
ATI Graphics
Boot Camp control panel for Microsoft Windows
Boot Camp System Task Notification item (System Tray)
Broadcom Wireless
Intel Chipset Software
Intel Integrated Graphics
iSight Camera
Marvel Yukon Ethernet
nVidia Graphics
Cirrus Logic Audio
Realtek Audio
SigmaTel Audio
Startup Disk control panel for Microsoft Windows

• Boot Camp 4.0, OS X Lion: Frequently asked questions
• Boot Camp 3.0, Mac OS X 10.6: Frequently asked questions