I had the same conundrum you mentioned. I wanted to be able to play impromptu and just jam with friends on guitar. A few weeks ago I was playing my trumpet with a straight mute with two friends on guitar while about 15-20 people were conversing in the same room, some only a few feet away. A cup, Harmon, or bubble mute would be even quieter. A stemless Harmon is amazingly quiet. I have a 90-year old King Liberty that I carry around at all times with just a stone-lined straight mute, and yesterday I played accompaniment on trumpet for a few people singing hymns. It was rather unorthodox, but it worked. Having said that, I know nothing about the french horn, except I've been led to believe it's a completely different animal than a trumpet.