0. Credit to virserg for doing the actual work:

    https://discourse.osmc.tv/t/easter-update-music-visualization-not-working-any-more/79468/10

1. Download the custom repository manifest to your pi:

    curl 'https://tri.jahschwa.com/link/spectrum' -o spectrum.zip

2. On your TV navigate to:

    Settings > Add-ons > Install from zip file

    IF PROMPTED ABOUT SECURITY / UNKNOWN SOURCES:

        2a. Choose "Settings"

        2b. Enable "Unknown sources" then choose "Yes"

        2c. Go back once then choose "Install from zip file" again

3. Navigate to the zip, by default it would be at:

    Home folder > spectrum.zip

4. Now choose "Install from repository"

5. Navigate:

    Virserg Repository > Look and Feel > Visualization > Spectrum

5. Choose "Install"

6. Choose "Spectrum" again then choose "Use"

7. Profit