Takes about 3 minutes and will make your Diablo 3 experience more enjoyable if you're a hardcore fan of Diablo 1 and/or 2. I can't stand the epic orchestra/lack of music in D3, so I just use my favorite tracks from the previous game. My favorites being Dungeon (D1), Caves (D1), Leoric (D2), Harem (D2), Monestary (D2), and of course Tristram from both games.
You need the soundtracks, of course. This is not perfect, but it does the trick.