Hi there !
So as usual @robin saves my life with that tip 
Now i can use my Bluetooth speaker UE BOOM 2 with Sonic Pi on the latest Raspberry OS (released on 2020-12-02).
So a little (yellow) summary (fab4 joke)
Note that i’ve installed this version of Raspberry Os : https://downloads.raspberrypi.org/raspios_armhf/images/raspios_armhf-2020-12-04/2020-12-02-raspios-buster-armhf.zip. Flash the img on a sd card then insert into your raspberry pi.
Then install the Robin package for raspberrypi OS available on the official website https://sonic-pi.net/files/releases/v3.2.2/sonic-pi_3.2.2_4_armhf.deb
Then follow the instructions given by @robin.newman :
sudo apt install pulseaudio-module-jack
wget https://gist.githubusercontent.com/rbnpi/8812203f5c9a995620bed9ce3a3c6a20/raw/30da158af589a011bafc26c9d19ad007a68d58ca/scsynthexternal.rb
sudo mv /opt/sonic-pi/app/server/ruby/lib/sonicpi/scsynthexternal.rb /opt/sonic-pi/app/server/ruby/lib/sonicpi/scsynthexternal.rb.bak
sudo cp scsynthexternal.rb /opt/sonic-pi/app/server/ruby/https://downloads.raspberrypi.org/raspios_armhf/images/raspios_armhf-2020-12-04/2020-12-02-raspios-buster-armhf.ziplib/sonicpi/scsynthexternal.rb
sonic-pi
Cheers