The usual argument is "I lead a very stressful life; if I'm going to continue with this I need a place where everything is good and nothing bugs me so I can go off and recharge". That is usually a good enough argument for nice speakers and a good TV.

Failing that, you can always try the "but we'll socialize more if I have a nice home theater" ploy.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8