I suggest you add the car instead of replacing an existing one. There's a tutorial on GTAF, don't remember exactly where since I've memorized the process since San Andreas...
Anyway you can change the car's sound in 2 ways:
1. Changing the actual sound, don't know how to do this one for IV yet.
2. Changing the car's ID in the vehicle data file (forgot what it's called by default, I use a self-made one so...), replace it with ''sultanrs'' or whatever the ID for the Sultan RS was.
EDIT: Fount that tut, full of mistakes though so be careful:
http://www.gtaforums.com/index.php?showtopic=420911
The vehicle data file is ''newveh.ide''.