Undo the long screws in the plastic cover. As
@pauljygrant said there may also be screws holding them from the inside (Inside cupboards etc).
On our expanda I was able to pull the top of the "skirting board" away enough to hook the speaker cable with a bit of wire and feed it through the ends into tunnel.
Also had to get the cables in behind the shower cubicle...... And NO WAY was I going to remove that!!!!!
Ended up getting
THIS out of the shed and had the cables located to stereo withing ten minutes

Best $3.95 I ever spent!