Adapted from https://ubuntuforums.org/showthread.php?t=2263834&s=21da51e852349f734144425885d56ac9&p=13221570#post13221570
Thank you very much to the author "Toz"

1. Install some required packages:
sudo apt-get install gnome-session-canberra sox

2. Download the sound theme

3. Extract the file:
tar xzvf Smooth.tar.gz

4. Copy the extracted content ("Smooth" folder) to your sounds directory:
 /.local/share/sounds
(create the "sounds" directory if it does not exist yet)

5. Enable sound events and feedback sounds.

In example in Mate go to Control Center --> Sounds and enable them there; in Xfce it is done by entering in the terminal:
xfconf-query -c xsettings -p /Net/EnableEventSounds -s true
xfconf-query -c xsettings -p /Net/EnableInputFeedbackSounds -s true


6. Set "Smooth" as the default sound theme

In example in Mate go to Control Center --> Sounds and set it there; in Xfce it is done by entering in the terminal:
xfconf-query -c xsettings -p /Net/SoundThemeName -s "Smooth"

7. Setup the necessary environment variable. Add to the end of ~/.profile:
Code:

GTK_MODULES="$GTK_MODULES:canberra-gtk-module"
export GTK_MODULES

That should be it. Perhaps a logout is needed before it starts working.

--//--

8. For a startup sound, create a new autostart application (Settings Manager or Control Center >> Session and startup >> Application autostart) with the following parameters:
- Name = Login Sound
- Command = canberra-gtk-play -f ~/.local/share/sounds/Smooth/stereo/desktop-login.oga

9. For logout sound, you need override xfce4-session. To do so, with root privliges, create the file /usr/local/bin/xfce4-session with the following content:

#!/bin/bash

# run the real xfce4-session executable
/usr/bin/xfce4-session

# on exit, run my stuff
canberra-gtk-play -f ~/.local/share/sounds/Smooth/stereo/desktop-logout.oga

...and make this file executable:
sudo chmod +x /usr/local/bin/xfce4-session
