I managed to locate all four movements of this trio with three giants of their respective instruments.
The first movement in organized A-B-A-B-A, and its sad and rather contemplative. At 2:25 the B section comes in at a faster clip in G minor. The use of a seventh chord at 3:42 is absolutely chilling. The A section returns in abbreviated form at 4:03 with a change in leadership among the instruments. The B section returns at 5:15, but this time in B-flat minor. When the seventh returns at 5:55, youre ready. The A section arrives at 6:19, but in G-flat Major. This is only a diversion because Brahms brings you back to the home key of E-flat at 7:15. He winds it down slowly with an end that is not so much happy, but resigned.
Brahms: Trio for Piano, Violin and Horn, Op. 40, first movement
In second position, Brahms places a scherzo in E-flat Major, and now its time for fun. At 1:00 he goes into the remote key of B Major in the kind of seamless modulation (key change) that only Brahms could get away with. At 1:44 its back to E-flat. The middle trio section of the scherzo shifts to A-flat minor, and it once again becomes pensive. At 4:45 the scherzo returns.
At the discussion with the musicians at the Seattle Chamber Music Festival about the slow movements of Brahms, this one came up as one of the greats. Its marked slow and sad, and its like coming home to an empty house after the funeral of your spouse and children. Its a three handkerchief piece. It starts in E-flat minor, and at 2:00 the horn leads into a short and slow fugue. This in turns picks up speed to a passage at 3:18 that builds to a return of the first subject at 4:08. At 6:45, grief and anguish set in, and also rage. At 7:17 the emotional numbness returns, and it ends as bleakly as it began.
Brahms needs to dispel the sadness, and now he writes the only movement in the piece that is in sonata format. Having avoided hunting horn calls, he writes a movement where the horn line is almost nothing but hunting horn calls. Its a fox hunt set to music.
The first subject is full of tallyho and horses in motion. At 0:44 the second subject comes in and features a short passage at 1:25 that sounds like theyve stopped to water the horses. The exposition repeat begins at 1:43. The development starts at 3:15, and the recapitulation comes at 4:29 where were back to full gallop. After watering the horses at 6:53, he races into the coda.