No sound on the PC monitor (Asus MG28UQ)?

I ordered about a year ago designated monitor from Asus. The sound ran smoothly for the time being, both the built-in speakers and the playback via Aux on a small Phillips system. For some time, however, neither the speakers nor the playback function by Aux (although here, unlike the speakers, a sound is heard, this is hardly noticeable despite full volume). When I received a new model after my complaint, I had the same problem again. Of course, I've checked all the cables several times (Aux, HDMI, Displayport, power cable, system, boxes work fine on other devices). I really do not have the slightest idea what it could be (monitor on XBOX and PC run, but does not work on both devices).

So under Windows you can set yes, over which device the sound is to be output. Did you set that right?

Yeah, I checked all the settings. In addition, the sound is on the system yes, just extremely quiet, which was not the case before.