Doesn't the NVR offer "email on event"? Not nearly as much fun, I know.
This might also be a job for "IfThisThenThat"... IfTTT.com... with the NVR sending a trigger, maybe via an Arduino "listening" for a beep, and small web servers in the differing rooms waiting for a command to beep. You could also code the "listener" Arduino to send the "you need to beep" command directly to the beeping Arduinos. Each node COULD, I KNOW, be done with a $15 Sparkfun ESP8266 "Thing"... and probably, if you've got a bit of ESP8266 knowledge, with even cheaper ESP8266s.
For a simple "remote beep", there are probably simpler solutions.... but for the other things you want, the ESPs, or something similar, will probably be needed.
"Doesn't the NVR offer "email on event"? Not nearly as much fun, I know."
Yes it has. But the email are not in real time if you are using Gmail SMTP forwarder. Unless you pay for a private forwarding, there is also a limit and Gmail often stop forwarding. There is a 1-2 minutes delay as you can imagine with Gmail.
I've already ordered stuff, it's in the mail including sound sensor to catch the Warning Beep from the NVR and translate it to data for the Arduino.
I've got CAT6 all over the house... for the hard to reach places I will use the ESP8266. The one I got, are they OK? I know you suggested the $15 Sparkfun ESP8266 but the one I got are like 6$. Is there any drawbacks?
Here is what I've got so far:
UNO R3 MEGA328P ATMEGA16U2 Development board for Arduino+ USB Cable SP
2016 New DC 5V 4-Mechanical Channel Relays Shield Module For Arduino UNO R3 XBEE
Breadboard Power Supply Kits with 65pcs Jumper Wire Cable For Arduino Project MA
Electronic Parts Pack KIT for ARDUINO Component Resistors Switch Button Top
1602 16x2 Character LCD Display Module HD44780 Controller Blue Arduino
ESP-12E ESP8266 UART WIFI Wireless Shield for Arduino UNO R3
Ethernet Shield W5100 For Arduino Main Board 2009 UNO ATMega 328 1280 MEGA2560
Arduino DS3231 AT24C32 IIC Module Precision RTC Real Time Clock Quare Memory
I doubt there's anything "wrong" with your ESP8266s... I just appreciated the hand-holding that I got at Sparkfun. Of course, much of what they say for the users of THEIR 8266 may apply to at least some aspects of using ANY 8266.
I also like knowing that if I order from Sparkfun my credit card is safe, and delivery will be reasonably expeditious, and not involve snafus with customs checks of things coming from abroad.