The setup. The scotch tape around the headphone cable is to keep it neat for the picture.
The components. Radio, audio cable, voice recorder and headphones.
The 2.5 mm goes into speaker out of radio 3.5mm goes into voice recorder. I got the 2.5mm cable from an old TI-83 calculator data transfer cable.
Inside the cable I have a 10K ohm resister to bring the audio level down to something the microphone input on the voice recorder can handle without getting overloaded and recording very distorted audio. Depending on your recorder you may need to experiment with different resistors to get the right level so it doesn’t overload your recorder. I also run the mono audio from the radio to both the left and right channels for the voice recorder. This way I can hear the audio in both earphones.
(note the resistors pictured are not 10K ohm. Just for demonstration).
No splitters are necessary as I monitor the radio audio from the the headphone output of the voice recorder. It does have to be recording for me to get audio, so if for some reason the battery in the recorder died I wouldn’t be able to use this setup.