Terry Pinnell wrote:
Could someone remind me of the simple test involving placing 'left'
and 'right' speakers close together facing each other please? Am I
right in recalling that they are in correct 'phase' when the volume
sounds reduced/muffled? Or is it the reverse?

(I have some cable extensions to do on the speakers that came with my
new PC, and both wires are identical, so I want to be sure I get it
right.)

Terry,
Cable with absolutely no polarity identification is unusual, though not
impossible to find. In some cases the identification is not obvious and
can be something like a rib on one side that may just look like a flash
from a moulding process.
Roger Thorpe