Thanks to all - perhaps these speakers are sealed and I need to look for alternatives. Great observation by Mickster - the surface marking is much more evident in the photograph than in reality and may indeed be the sign of ultrasonic welding.
I must say that as a newcomer to this forum I have been totally impressed by the level of assistance. I am a long experienced electronic engineer and have applied PICs (all Series) over many years. I look forward to maybe contributing to some threads in the future.
atferrari, if you are the same person, I remember you well from the Microchip Forums many years ago! (I posted under a different user name at that time).
Regards to all.