Lugubrious means, doleful, mornful or dismal. It sounds as though it should be something fun, doesn't it?
As far as I can recall I've only ever come across it in relation to speech, but characters needn't just speak lugubriously, the could show their inner lugubriousness through a sad expression and dejected manner.
Much as I like the sound of foghorns, I think it would be fair to describe the noise they make as lugubrious.
Thanks to Beatrice!!!!