You mean all the way up the wall to where the speaker is to be mounted
and then plaster over the conduit, or just a little bit where the cable
is to reappear and then have the rest exposed ??

The former would be best. The latter better than chopping the skirting.

as it a leasehold flat and I dont want to start chopping up the walls

You can normally do this to a leasehold flat. Or did you mean you are
renting?

Christian.