The source code is available on Fe-Pi GitHub repository: https://github.com/Fe-Pi/linux.
First, you would download the kernel sources using the following command:
git clone --depth=1 https://github.com/Fe-Pi/linux.git
Next, follow the instructions that are published on the Raspberry Pi organization here: Kernel Building
Configuring Fe-Pi Audio is done using Device Tree overlays.
To do this, you must edit /boot/config.txt and add the following lines...
If you want the on-board Raspberry Pi sound devices(HDMI, 3.5mm jack) to be disabled then change...
dtparam=audio=on to dtparam=audio=off
Precompiled Kernel and Modules
If you need to do either an apt-get upgrade, apt-get dist-upgrade or rpi-update, files with Fe-Pi Audio support will be replaced, and the sound card will no longer work.
To restore the files (kernel and modules) with Fe-Pi Audio support, download the latest file from here: https://github.com/Fe-Pi/binaries.
To install, copy the file to microSD and use the following command:
sudo tar -xzvf linux-4.4.x.tar.gz -C /
(Note:replace linux-4.4.x.tar.gz with version downloaded)