First thing to be aware of is that on these machines is that the cassette connection terminals are marked with where they go to on the recorder, so MIC on the computer is the output to tape and EAR on the computer is the input from tape. Actually both signals eventually go to / from the same pin on the ULA IC, but through a different combination of components.
I did some similar experiments with a small MP3 player, the sort of thing which runs on one AAA cell - Actually it was a Creative Zen Nano. Like most such devices the output was stereo and the first thing you shouldn't do with a stereo-output device is connect the left and right outputs together, so normally you'd only use one output or the other, plus GND, for the signal going out to the Timex / Sinclair EAR input.
In the case of the MP3 player above I found by experimentation that the input to the ZX81 (as was in my case) needed to be at least 0.7V peak to peak and one output channel alone could not generate a signal that big, so the first thing to do is to measure the maximum audio level out from your voice recorder if you have an oscilloscope or access to one. (I'm assuming your recorder is mono, with a mono output).
If you want to check out the Timex independently try using a laptop as the 'recorder' as they usually have Mic in / Audio out on one or two 3.5mm sockets and most come with a simple audio recorder 'App' such as Windows Soundrecorder, or you can use something like 'Audacity'. Again with the laptop, use only one of the two stereo output channels for output to the Timex and do not short the stereo outputs together.
