So to keep costs down I'm trying using the pi zero at the moment.

• Raspberry Pi Zero W with Soldered Header / Male Header - 40 Pin (2 x 20) - Straight [$19.37]
• JustBoom Amp Zero pHAT Maximum output is 30 W (RMS) 55W (Peak) [$33.21]
• SD card
• Adafruit FONA 808 - Mini Cellular GSM + GPS Breakout [£50]
• Slim Sticker-type GSM/Cellular Quad-Band Antenna - 3dBi uFL [£3.50]
• Visaton Cabinet Speaker, 30W nom, 50W max, 8Ω [£21.06x2]
• SIM
• Sealed enclosures for speakers

Details for JustBoom Amp Zero speaker power requirements :

8Ω Speaker

Amp Gain 26dB

Supply Voltage(V) 24

Peak Power(W) 55

Max Power(W) 30

Supply Power(W) 75

So that will be awkward for batteries! And I don't even know if that will be loud enough!