Hi everyone, thanks for your input.
In the "advanced" tab, I have the "simultaneous record/playback" option checked.
In the "drivers" tab, the field "Input Drivers" have only one item, namely "1. Microphone (Realtek high definition)", and I can leave it either highlighted or not. I have it highlighted, which I assume means selected. In the field "Output Drivers" I have two options, "1.Speakers (Realtek High Definition)" and "2.Realtek Digital Output (Realtek)". I have "1.Speakers (Realtek High Definition)" highlighted.
In the Windows mixer "recording properties" tab I have "Microphone (Realtek high definition audio) listed as "working", "Line In (Realtek high definition audio) listed as "not plugged in" - quite natural, as I only have an audio input jack connected to the mic input - and "Stereo Mix (Realtek high definition audio)" listed as "disabled".
The situation remains the same: any monitored audio track inevitably ends up recorded with any new audio track I try to record. I wish there was a dedicated control to select what gets input to the recording tracks... Perhaps it is poorly named and I haven't identified it yet? I also wonder whether it lies in Cakewalk itself or somewhere in the Windows Vista configuration.