The CDV-700 headphone is a 10k ohm electromagnetic monaural clicker. The pulse from the audio circuit simply causes a thin metal disk to flex back and forth to make the sound. These headphones are relatively hard to find and expensive compared to a regular set of stereo headphones you would use on your iPod or other music device.
The female audio plug on the end of the CDV-700 headphone is an Amphenol 2501F. These are also hard to find and expensive. They were a common microphone adapter back in the days before XLR became the standard.
Step 1: Repairing the Original Headphones
The only simple fix for the original headphones aside from replacing/resoldering the connector is replacing the inside with a piezo clicker.
This is an exact fit once you cut off the two mounting tabs, but Radio Shack is going away. This appears to be the same item sold from China without the brand name
The rated voltage is 12v, which is the same as the pulse height of a properly working CDV-700. These are polarized. The red wire goes to the center pin of the headphone adapter.
Step 2: A Better Fix
As it happens, an Amphenol 2501F has the same diameter and threads as a PL259 antenna connector. The PL259 has a longer center pin, but that is easily cut down and smoothed over with a touch of solder. In the picture above the 2 connectors on the left in the picture are 2501F and the 2 on the right are PL259's cut down and soldered to make them a functional substitute. This works but still has one drawback. The PL259 was an antenna connector. It was made to twist on or crimp the braided shield of the antenna. There is no proper place to solder the return wire to use it for a headphone. The 2501F has a spring and a set screw to attach the return wire. I have drilled small hole in the end of a PL259 adapter or wrapped a spring around the end to make the connection, but there is a much simpler fix.
The black and gold adapter is an RCA male to 1/8" mono female.
The other adapter is a PL259 to RCA female which is common and cheap. This was a closeout, but there are lots more like it. These were less than $1 each.
If you shorten the center pin of the PL259 and solder it smooth the second adapter plugs in directly and your headphone into that. If you want to sample the output of your CDV-700 to a computer, remember that the pulse height is about 12v. http://en.wikipedia.org/wiki/Voltage_divider You will need a voltage divider in between to reduce the height to about 1v so you don't damage or destroy your sound input card.
I intended to put a 820 ohm resistor in between the 2 adapters, but the center pin of the PL259 adapter was solid. Some members of the CDV-700 Yahoo group said there might be models that would be damaged if there was not enough impedance on the headphone circuit. I haven't had any trouble that way, but very low impedance headphones will pass through circuit noise. If you use 32 or 64 ohm headphones you shouldn't hear the whine of a Lionel 6B circuit, which is the noisiest of the later models. Vic 6 models have separate audio circuits and are quite a bit quieter than a Lionel 6B.