I discovered that a few days ago and I haven't seen any topic on PinSide so here it is.
A new sound board for WPC89 seems to be in the work here: http://www.pinsound.org/
Here is an excerpt from the site:
PinSound is a new sound board for pinball machines. For now, the PinSound kit is compatible with WPC89 pinballs.
- plug and play sound board
- full new amp 2.1 to provide the best sound for original speakers
- harness wires included to upgrade from mono to stereo system
- each sound available on memory card (can be easily updated with a computer)
- possibility of mix between original sounds and high quality sounds
- possibility of applying filters individually (if you want specific sounds to be louder than others, for example)
- multitracks (the PinSound board can play many songs at the same time, unlike the WPC89 sound board, which is limited)
- headset connector
- made in France