I checked out the schematic you posted, and at first the circuit around the 5V point was not make any sense. I believe it is rather a 34.5V point ! ( A2 and A5 are definitely 8.6V and -8.6V according to this schematic )
IMHO, your best option is to use the +35V present across the C5 capacitor ( it is the leftmost large red circle on the drawing ). You can find the negative side ( 0V ) by identifying the large white band on the capacitor.
Then use an isolated DC/DC to get a clean isolated 5V (you don’t technically need an isolated supply, but it makes things a lot easier to avoid noise and some issues with different reference potentials).
I would recommend the TMR 4-4811WI, as it accepts inputs from 18 to 75V.
Last advice : You might want to find a bluetooth module with a good quality audio output. Many of them are sub par, and would probably sound like a disappointment on those speakers.
Best luck with your project !