i did diy sound proofing for my last car, very extensive some more. 4 doors, boot and wheel well, all wheel arcs and floor pan (yes i remove all the seats).
not that difficult, but very tiring and exhaustive. my previous ride is a Mazda 2 SP. few months after i done my SP, i sold off the car for my current latio..... faintz.
the only thing i have not done is the roof, cause it gets noisy only when it rains very heavily. but the chances that its raining very heavily and i am on the road is slim