Many thanks for your help mate

Also notice the existing surrounds were made up of two half-circles pasted together -- must not have had the correct diameter on hand.  You can see the seams at 9:00 and 3:00. And it looks like two (2!) repair efforts were made, given different colors of dried glue.

So I hope somebody can tell you if the part number substitution buying whole drivers is OK.  Best choice given that mess.  But if not, read on...

[Edit: Before wasting time on foam, double-check with an ohmmeter that the speakers are about the same resistance.  A reading of infinity (open, split wire) or of zero (short, fused wire) is obviously bad, and not worth wasting more time on.  Nobody mentioned this prior because it's really unlikely, but given that photo, anything is possible!]

As to the solvents, what you use will depend on how crummy the glue from previous attempts turns out to be.  First try up to 91% isopropyl alcohol, and a long fingernail is the finest tool you have or will need.  Failing that, a stronger solvent, like "denatured alcohol" or (USA) Goof-Off™, don't get the stuff on your skin -- you should probably use the *back* edge of an X-Acto knife straight blade (actually a triangle).  The blade would cut the speaker cone, but the squared-off rear usually catches the glue without ripping into the driver.

If the previous attempt was particularly horrible, the driver might be ruined anyway.  Once you remove the old foam, examine the remaining, probably now jagged, edge versus the overlap of the new foam.  Also follow the instructions with the re-foam kits to press/piston in&out to make sure the driver coil isn't rubbing against the magnet or frame before you waste time gluing.  Not sure why being coy about where to order parts from Holland, but here in the USA: https://www.simplyspeakers.com/bang-olufsen-speaker-repair-foam-edge-kits.html (International shipping might be expensive, but the basic parts are cheap.)

Thank you for your help mate.

To be honest I think I’ll just buy new drivers. Had these speakers since the 90s, and want them to sound like new.
One thing I noticed which I’ll recheck later, was that there wasn’t much sound coming from the tweeters. If there’s a n issue with them, that would also explain the muffled sound.
I’ll check the 8pin din sockets also

Ordered 4 new drivers from Austria.

Hopefully this fixes the muffled sound.

Another thing I noticed was that the tweeters would occasionally cut out. I’ve checked the connections, and they all seem ok.