I'm watching Abhi To Main Jawaan on Hindi "TV Asia" through Dish Networks now... it's one of my favorite shows, always rewarding. I don't understand much of the narration, but host Shiraz Sharif lives in New Jersey, and has access to many movies from the 1940s and 1950s.
The emphasis is on melody... there's nowhere near as much harmonic development as we saw in Hollywood movies of the 1930s and 1940s. But the main instrument is dual-drum tablas, laying down the meter for the vocals.
I rarely get the chance to watch full classic movies, because at 3 hours a pop, I just don't have the time. These vignettes Sharif presents are just musical segments embedded within a larger entertainment piece -- they're only part of what the audience sees -- but I do like what I see in this show, very much.
Newer movies aren't as much fun for me... there's a lot more emphasis on electric guitars, and western-style beats that pound you in the head, without the subtlety found in these pre-Beatles movies.
For melodic instruments, there are often string sections or harmoniums to lay in a background of tonality without meter. Flutes, mandolins, harmonicas, sitars, sarods, more all take melodic variations during vocal pauses.
There's often a switching of modality, retaining the same tonal center but with a different scale for certain lines.
For tune structure itself, there seems to be quite a bit of AABB structure... a vocal line is repeated very often, from what I can tell.
Another big difference: the whole thing is lipsynched, there isn't the "live performance" aspect which I'm used to. First the music is recorded, and then the actors act it. It's not an integrated experience. Maybe that's why I like the older movies more, because there wasn't as much time for such a discrepancy to develop between the sound and the eventual visual experience.