This is just too much lack of info...
Better title next time, because your title is just a attention-wanting title which lead to NO good at all on a Alpha feedback site.

More info too, like:

1. Only occur during moment or all the time?
2. Did it go away when you removed the backpack?
3. Can you repeat the glitch after restart

Tons of small questions you need to answer to be any help at ALL for the devs.

Altough I dont know if the sounds are properly set up and if ambient sounds affects the enemies hearing.

BUT firing a silenced rifle is audible from 100m and I believe even 200m with on a field. Especially assault rifles.

This issue it related to ports and firewall security I believe, you had to open some more ports that the game itself required. As stated above, a problem also known in ARMA 2

I had kind of this problem before, but that was I through steam opted for the DEV Alpha and instead of last stable alpha.

So if you have opted for the DEV alpha try Turing it off