How do I split a mains power source between two devices
It's best to avoid working in mains power at all: Safety; ease of getting UL/CSA listing; and ease of changing power supplies if you change countries. 

You may have noticed that almost every small machine uses a wall wart power supply.  This is why.  They can focus on their low-voltage gadget and not worry about also designing a mains voltage power supply and getting that listed.   The wall-warts are commodity items, and are already UL listed.  

Lately the trend is USB power, but that only works if your load is less than 5-12 watts.   Yours is more.  

So first... Choose  a speaker module that runs on the same voltage (5.1) as the Raspberry Pi.  This won't be hard;  5V is a popular voltage. 

Then total the current requirements for both Pi and speaker.   

Then look for a wall-wart that outputs the correct voltage and at least the total current, preferably a bit more.   Too much current is good, too much voltage is a problem.  

Then search for a connector socket which fits the plug on the wall-wart, and fit that into the back of your chassis.  Split power from there to both devices.