This isn't a review. Just a few quick observations on A R Rahman's musical technique.
When I started watching Slumdog Millionaire, I was trying to guess whether the soundtrack and background music was by A R Rahman. I didn't know because I saw the film without reading or following too much about it. I wanted to watch it with no preconceived notions. The opening chase song O...Saya had a characteristic Rahman feel - expansive sound, ARR-sounding vocals and a train beat to go with the train visual. But M.I.A's arrival on vocals threw me off the scent.
Then I got so caught up in the film that I stopped thinking about it. A compliment for good film technique (and background music ) is that it doesn't draw attention to itself outside the context of the film while you're watching. The music fits the film perfectly.
During the song-and-dance end credits though, there was a point when Sukhvinder starts singing the chorus Jai Ho. He starts by himself with a backing layer of keyboards. Sometime into the second refrain, an additional layer of music kicks in, making for a goosebump moment.
At that point (maybe a minute before Rahman's name pops up in the credits), I had my answer. No one quite layers sound for effect like he does. He draws you out slowly, adding layer on layer, preparing you for a final assault and a pitch-perfect crescendo.
Another interesting technique that I noticed is the use of voice as sound. While his liking for fresh playback voices is well-known, what is probably not appreciated is how those voices add to the 'sound' and feel of a song. An example that immediately comes to mind is the female playback singing in Pappu Can't Dance! and the use of Vasundhara Das for barely two lines in Kahin To*. The use of Mahalaxmi Iyer( or is it Tanvi Shah?) on vocals in Jai Ho does that perfectly. Complimenting Sukhvinder's earthy voice and the Spanish-sounding chorus, that voice breaks through and registers on a different level. I'd have expected Alka Yagnik or someone similar to sing that exuberant love song but he surprises us, defying our musical expectations to come through with something bordering on the sublime.
*Vishal-Shekhar's use of Preeti and Pinky in Bluffmaster for Say Na Say Na qualifies too