[Dave Moll]

They run on 2x No.12 batteries, for which a pair of either "C" or "D" cells are a suitable substitute.

Regarding the problem of the quiet sound, I take it that the handsets have been swapped as a whole (by unplugging them at the four-pronged plugs). If this is the case, the problem may either lie in the receiver (earpiece) of the quiet one, or (more likely) the transmitter (microphone) of the other one.

Is the sound simply quiet or is it also distorted? The latter would confirm the problem is with the transmitter, owing to the carbon granules not responding to the pressure waves of the speech. This can sometimes be improved by taking out the transmitter inset (by inserting a pointed tool into the small hole in the side of the mouthpiece cover while twisting this cover slightly anti-clockwise) and giving the inset a few firm (but fairly gentle) taps against a solid surface to free any clogged granules.

If a conclusion cannot be reached as described above, the way to confirm whether the problem lies with a transmitter or a receiver is to swap the transmitter insets between the two handsets and see whether the problem migrates.

Edit: for some reason, Richard's second reply didn't appear until I posted mine.