If 'they' do not utilize a microphone but enter the circuitry of the computer directly, you should be able to record 'them' with the sound card of the computer.
In Windows the sound card may initially be hidden from view and disabled, right click on the list of audio recording devices and select Show Disabled Devices.
Enable the sound card and if possible you could set it as the default device or choose the sound card as the device of choice in a recording program.
Make a recording, it will be very silent except for any audio playing through the computer and system sounds (all of which of course you would prevent from happening) and supposedly any 'discarnate voices'.
Such an audio file is not absolutely silent though, once normalized, you will hear background noise but no interfering ambient noises.
You should try that.
An example of normalized sound card audio is attached.