To install the sounds, you must connect to the Asterisk server via a SSH client as root.
Once inside the server, create the destination directory for audio files.
mkdir /var/lib/asterisk/sounds/en
Download the core package and extra
cd /var/lib/asterisk/sounds/es-ar
wget -O core.zip https://www.asterisksounds.org:443/en/download/asterisk-sounds-core-en-sln16.zip
wget -O extra.zip https://www.asterisksounds.org:443/en/download/asterisk-sounds-extra-en-sln16.zip
unzip core.zip
unzip extra.zip
chown -R asterisk.asterisk /var/lib/asterisk/sounds/en
find /var/lib/asterisk/sounds/en -type d -exec chmod 0775 {} \;
Now, you need to set the new language in the desired internal or in the general context of the protocol (SIP, IAX2...) where you want to use the new sounds.
FreePBX configuration
To use the sounds using FreePBX, just modify the language variable parameter in Asterisk SIP Settings.
How Asterisk Searches for Sound Prompts Based on Channel Language
Each channel in Asterisk can be assigned a language by the channel driver. The channel's language code is split, piece by piece (separated by underscores), and used to build paths to look for sound prompts. Asterisk then uses the first file that is found.
This means that if we set the language to en_GB_female_BT, for example, Asterisk would search for files in:
.../sounds/en_GB_female_BT .../sounds/en_GB_female .../sounds/en_GB .../sounds/en .../sounds
This scheme makes it easy to add new sound prompts for various language variants, while falling back to a more general prompt if there is no prompt recorded in the more specific variant.