For softer and more stock engine sounds install the following files in the following way:

In OpenIV go to: 
Grand Theft Auto V\mods\update\x64\dlcpacks\toysupmk4\dlc.rpf\x64\audio
and put 
toysupmk4_game.dat151.rel
toysupmk4_sounds.dat54.rel
into this folder

then go to:
Grand Theft Auto V\mods\update\x64\dlcpacks\toysupmk4\dlc.rpf\x64\audio\sfx\dlc_toysupmk4
and put
toysupmk4.awc
into this folder

Ta-Da! you have installed the alternative sound