I can only speak for the LG 34UC97-S
1) Will all the Display Ports being able to send the audio from my GTX 970 mini to the monitor speakers?
Yup, the DP port will carry audio to the monitor.
2) I get some feedback that the LG 34UC97-S HDMI 2.0 only provide 50hz, can someone confirm me that?
Nope, it does 60hz, not sure where you got 50 from...
7) Can someone give me some feedback about the speakers of the monitors I talk about?
I find that the LG 34UC97-S is a awesome monitor, really love it. I mainly use it for work/productivity related activities as its not the best to game on but it doesn't perform bad when I have played the few odd games (BF4, H1Z1, Boarderlands). I Also have the LG 34UM95 at home and I find that the colour and the picture quality is much better on the LG 34UC97-S.
I don't think that the LG 34UC97-S is vesa compatible with out buying an extra mount for the back.