Yeah so. I used to have a 5.1 home theater setup before I was a Mustachian. I am not a 100% Mustachian now either. But, I am trying to. Fortunately I did not squander away all my money and I am track with my retirement plan and I'm a generally happy person. I sold my HT setup when my DD was born 4 years ago. Now that she is older, I am thinking of getting back into having a HT again. It would cost me $3500 for a really good 5.1.4 setup. I am split. Part of me says, go for it. I worked for it. This is something that I enjoy for years to come. I watch all my movies at home. We don't go to the cinema. Part of me says, put it towards retirement.
You can also say you worked for a fancy car, and a big-ass house, and to send your kids to private school, and to take expensive vacations. Oh: and you also worked in order to free Future You from the need to keep working. But the proper conjunction is "or," not "and." Which of those is the most important to you?
Look, if this is your big "thing," and you are ok with how it will impact your FIRE date*, then your choice. But, damn, that is a shit-ton of money for sound. What about trying cheaper options first? Or, you know, find another guy who is about to have a kid and take his system off his hands for a song.** :-)
*But don't fool yourself into thinking this is a one-time event. You are jumping back on the treadmill of fancy-schmancy cinema "experience." Your financial analysis should assume that you will "need" to replace and/or "upgrade" to a better model on a regular basis.
** Part of me thinks that there is only one home theater system, and it circulates from guy to guy as they are about to have kids. Kind of like fruitcake.