Something both onlookers and myself noticed is that while I'm playing theremin I'm often moving my mouth and throat as if I'm whistling or overtone singing. Some people where asking if I was somehow producing the sound singing or whistling after seeing me play. When practicing I often start off singing or humming on a comfortable pitch and matching it on the theremin, though when I try to keep on using my voice while playing theremin it soon gets confusing. I think what's happening is that my mouth and throat are subconsciously matching the tones I'm playing or intending to play with the formants/resonances that I use while overtone singing, playing jawsharp, yidaki (didjeridu), harmonica/melodica and bamboo flute.
Maybe whith plenty of practice it would be possible to use voice and theremin at the same time, first keeping a steady tone with one and varying the other, second moving simultaneously and third most challenging doing different but harmonious voices simultaneously. For now with a looper (Boss RC-202) I can combine voice (and other instruments) with theremin by alternating/overdubbing.
At the moment I'm not sure whether the subconscious subvocalising I tend to do while playing theremin is helpful or detrimental to my playing in the long run, on the one hand the subvocalising/formant matching may give me a connection to instruments/techniques I have been using in music and may give some feedback of feeling the resonances of the theremin sound as a resonance match in my mouth/throat and in that way may enhance the hand-hearing biofeedback loop with the tunable formant filters the mouth/tongue/throat function as. On the other hand it may be inefficient waste of energy/tension. I would be interested in the perspectives and experiences of other theremin players regarding subvocalising while playing and/or combining voice/singing with theremin and the feasibility of doing both simultaneously (one especially tricky part being that for singing there tends to be more body movement needed to do with breathing which must be somehow compensated with respect to body and hand position in the pitch field).