First you need to enable the corresponding driver for the SVX_Hat board. Edit /boot/config.txt as follows. Add or uncomment the following lines:

# Enable WM8731 codec
dtparam=i2c_arm=on
dtparam=i2s=on
dtoverlay=i2s-mmap
dtoverlay=rpi-proto

Optionally you can disable the onboard sound card by commenting the following line:

# Enable audio (loads snd_bcm2835)
#dtparam=audio=on

Save the file and reboot the Raspberry Pi to enable the SVX_Hat. After reboot open `alsamixer` (and select the second sound card in case you did not disable the onboard sound card). Make sure to unmute the channel named *Output Mixer HiFi*. Otherwise the card will not play any sound.

In order to record sound you may also need to enable the line in input in the capture controls.