Graham. has brought this to us :
I used thin twisted pairs salvaged from an unwanted CAT5 cable. Hiding
the wiring between floors by removing the backplane of the house and
tacking the wiring up the edges of the walls. I didn't attempt to
conceal the runs from the back to the centre of the ceilings, you may
have carpets etc.

As an alternative to the CAT5, you could use enamelled copper wire, as
used for motors and transformers. Get the type where the insulation
allows you to solder to them without scraping the enamel off first.
That is much thinner than the cores of CAT5.