Hi the FP3+ has ‘better’ cameras, and an updated core module to support the updated speaker.
The Fairphone 3+ was launched in August 2020.[15][16] It has upgraded front and rear cameras, improved audio, an increase in the amount of recycled plastic it contains (40% rather than 9%) and Android 10 “Queen Cake”.[15] The rear camera has a larger sensor, intelligent scene detection and a higher dynamic range.[17] The front camera has a bigger sensor and a higher dynamic range.[18] The new cameras and audio are alternatively available as modules that can be swapped in the original Fairphone 3.[15]
The audio improvements cannot be achieved purely by a speaker module upgrade. The speaker module is only for the rear/side speaker and output is only improved if the new core module for the FP3+ is used.[19]
Fairphone 3 - Wikipedia