I almost married a chorister three decades ago, and when she was out rehearsing I would put on the Ormandy recording of this piece and sing along with the baritone part.
At 2:00 the time signature switches to 3/4 and the key to C Major, and I loved singing the background baritone part that keeps the pulse.
When the Lacrimosa returns in 9/8 at 4:20, Berlioz makes use of the four separate brass bands in the balconies of the hall or church as brass expostulations during the choral work. At 4:45 he brings in the separate section of tympani, using them the same way he uses the brass bands.
At 6:45 he switches to A Major for the 3/4 section. But what a surprise awaits! At 7:18 there is a titanic battle between the chorus, insisting on E, and the brass section, insisting on D#. Its the battle between heaven and hell! And then Berlioz plays his trump card. At 8:28 he staggers the orchestra and chorus one-half beat off each other to take advantage of the echo in the hall. He ends it in a triumphal A Major, letting the sound taper off into a pianissimo.
Well, that is pretty awesome! :)
Who knew!