I don't have much experience from bare metal programming on the Pi, but I do know that is much like working with a MCU. You should think of it as if the Pi is an Arduino and your programming environment is a computer with cross compiler, kind of like the Arduino IDE. (just not an IDE in this case).
Following that analogy, there are no drivers to "install". Instead you need include libraries that support the hardware you intend to use. I guess you are aware of that as ethernet is not supported out-of-the-box for bare metal Pi, it is supported by the USPi library. The documentation for a library/driver should tell you how to use it. Or you'll need to read the source code and possible examples. And all configuration for WiFi must be done when writing the code. Or code must be written to allow configuration, like when using an ESP8266 with the Arduino IDE, in which case you must also provide a user interface to do the actual configuration.
As of February 2018 I'm not aware of any WiFi library/driver for bare metal Pi, any pointers or links are welcome! An obvious solution for bare metal gurus would be to port an existing library for ARM. For RPi 3 and Pi Zero W users that want to take advantage of the internal WiFi, there is a complicating factor: the Broadcom chip is not exactly well documented..
Now, I'm ramblin', but it seems to me you are crossing the creek to get water....
If you just want synchronized multi-room audio, there are other solutions that can take latency into account. Just google for it. Some of these work on the RPi too, and does not require you going bare metal.
On the other hand; if you want to educate yourself, you're up for an adventure!
