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.