Both are thriller/crime drama in a criminal/detective setting (not in all the episodes in the first responders). both solve new cases every episode, while also still having a main plot. there are few differences like in Voice it heavily focuses on crimes like murder and abduction while in the first responders it does have cases like murder and abduction but also suicide, and also in Voice the FL uses her supernatural hearing ability to save the victims, whilst in the first responders they use their own critical thinking and don't have supernatural abilities. overall both kdramas are similar in a lot of ways, and if you enjoyed one of them you'll definitely enjoy the other.

they both have similar genres: mystery thriller. and they also have a main plot while having smaller plots each couple episode that sometimes relates to the main plot.