No sound output from Spotify on 18.04

The simplest solution is to install pavucontrol sudo apt install pavucontrol. Then you launch it and the gui will let you chosse the output for each application.

Yes, I experienced a similar problem today but not just with Spotify.
In my case, no application was able to play the sound.
Only system sounds were coming.
This occured after I had plugged my laptop to my TV using HDMI cable and then unplugged it.

This is how I solved it.

1. Connect your PC back to the Device using HDMI port
2. Go to display and mirror the screens
3. Go to settings>sound.
4. Change the audio output to the HDMI device
5. Change the audio output back to the laptop/PC
6. Go to display and switch back to your laptop screen
7. Unplug the HDMI cable