As the old thread has been locked (wrong thread section), here is the V2.
Some engine sounds are wrong (recycled from Horizon 3 for the most part) and need to be updated. Most of those “wrong sounding” cars in Horizon 4 are right in Motorsport 7.
Older Honda Civics and CRX as well as Acura RSX and Integra share an awful VTEC sound that needs to be changed. Used to sound way better before FM7. Ferrari FXXK and Zonda R also have bad sounds that used to be better before Forza 7. Mazda RX8 and RX7 also used to sound better. The latter had pretty good sound in FH1 but FH2 and 3 were okay too.