There was an access hole already at one end of the room. When we had our new radiator installed (which was leaking mid-week — I had to tighten one of the nuts to stop it), the plumber cut another access hole at the other end. We simply had to run the cables through from one hole to the other one 5 metres away.
That wasn't easy. I had a 2-metre length of pipe I used to push a thick piece of old cable under the floorboards to a third, very small access-point mid-way between the others (where a radiator vertical pipe had been taken out) and then push it from there to the far hole. I used stiff cable so that it wouldn't get caught so easily on the rough concrete base. Once that was through, I tied some string to it and pulled it back. I then kept pulling on the string so I could make a loop. This enabled me to pull the TV and speaker cables from one hole to the other one at a time.
Amazingly, it worked. I was thinking I might have to drive a remote-controlled car underneath the floorboards or something, but for once my DIY improvisational skills were up to the task.
The hardest part of the whole exercise was finding somewhere that sold balls of string.