I just upgraded to High Sierra a few weeks ago and noticed an issue with the Messages app I did not have with Sierra. I no longer get an incoming message sound if Messages is the frontmost app. If Messages is in the background, it does make the Note sound as it always has, as selected in Messages preferences.
This is annoying because I have an external monitor and keep the Messages window at the left portion of it. Since I can scroll pages in web browsers and other apps on my main monitor without them necessarily being the frontmost application at the time, sometimes Messages stays in front and I do not realize a new text has been sent because I get no audio cue that it has.
As I mentioned, this is a change in behavior from Sierra. I hope it is simply a corrupted preference of some kind and not a new design decision from the Mothership.
