I am not expert, but old engine sound was more expressive and a lot lauder. If people really did not complain about that loud engine tone there was no reason to change it, I think. But maybe new cars are faster?
engines are still as loud of not louder, since there is no sound restriction in F1 and basically the engine exhausts are deristricted and nothing on their way.
What makes the sound different is the pitch which was much higher in the past, the engine are V6 in stead of V8 and V10 but more importantly, they are restricted in terms of RPM (16K only vs the 20K or even 22K in the V10 Era) and turbos increase pressure in the engine ...ect .
The new cars are slower, since the new regulation reduce aerodynamic capabilities of the cars and rules for fuel consumption /fuel limite, and engine limite has been added, there is one small saving grace that might add a bit of show, is the electric boost from kers system, the electric engine are much more powerfull, and the power system has been upgraded, so the boost might allow pilots to past each other