This is weird because I just used mine last night for the first time with the S7 Edge and it worked flawlessly...in fact even better than Note 7 in that the audio was in perfect sync with the video. With the Note 7 there was sometimes a very very slight offset...not enough to ruin the experience but it was there. HOWEVER, my connection did drop a few times. I was at my boyfriends and I have set up as a "my places" and one of the things that it does is turn off wifi...so I suspect the drop would happen because it would suddenly re-recognize that's where I was and turn off wifi. The mirror cast does something where it creates it's own wifi connection so I think it would turn that off because it knows wifi shouldn't be on at that location. Hopefully that makes sense.
So I'm very happy with the performance of the S7 Edge with the ScreenBeam Mini. I'm wondering, are you using the supplied HDMI cable extender, or your own HDMI cable? Because I'm wondering if swapping out the HDMI cable would fix your audio issue. Or trying a different port on the t.v. Or, lastly, rebooting the phone and reconnecting to the ScreenBeam. I used it last night with Netflix and it worked great.