You could get sound absorbing pads for placing the speakers / Sub on, but that would only somewhat deaden the contact between the speakers and the hard surface itself, and be of minimal assistance. If you’re concerned, try adjusting the level of volume at which you watch movies, and periodically check with your new neighbor. Once the building has been finished, there just isn’t a lot that can be done non-destructively for sound mitigation. 

Thanks

i have started looking at pads etc and as you say i will have to get to know the neighbor and see what it sounds like their side.

i cant decide if having the TV at the joined wall facing away would be better  or try it on the non joined wall but facing the neighbor

In talking with the neighbor, you may want to some sort of understanding of their schedule, If they have young children, perhaps don’t play the system loud during nap time or after bed time.  Maybe save movie night for the weekends.

I would keep the sub away from the joining wall. As far as the location of the Arc, higher frequencies are directional so I think you’re better off with the TV on the joined wall.  However, if the other wall is a good distance away from the joined wall, you might be better off that way.

Adding another layer of drywall is a quick and relatively inexpensive way to reduce transmission. Stagger the seams. This is helpful, but not quite state of the art. Do some reading.

 Invite them over for a movie & popcorn.

An additional layer of drywall is good, putting up a layer of sound absorbing material between them is even better and doesn’t add much to the cost.

https://www.dynamat.com/applications-industries/home-architectural/

It has been requested in the past, but not offered yet, that the Night-Sound option be given automatic on and off times for situations like yours.

Thanks for all the good advice