I have an "oooo" but

[Cool Hand Lurker]

Don't oil "too much" and be careful to not tighten on your cover "too tight" as it affects your horn's sound. I use 10 W sewing machine oil on the felts in Long Island winter and 30 W straight oil in summer. You said you left "all the settings" as was? If that included the adjusting screw, turn it in and then out at the engine RPMs you expect to do your Ah HOO Gah in.
Good Luck !

The horn is electric. How would the engine RPM affect it?

 

[Mitch//pa]

The horn is electric. How would the engine RPM affect it?

charge rate as well as ambient temperature will affect the sound
especially on an unregulated charging system

 

[Bruce Adams]

Your voltage goes WAY up with the engine running compared to with a stopped engine. Ever look at how your headlights go dim when you turn the car off in the garage with lights on. Do what I suggested since you will find a great difference in the horn's sound with the engine on.