If you have lots of mud could you not decide where to put your speakers cut the drywall, put in proper bracing behind, reinstall the drywall, mud, and then repaint? I would as the QS8's are heavy and I wouldn't want one to come crashing down. Especially with my kids banging into the walls occasionally.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70