Whilst the meaning/intention of the voice messages may be "mysterious", the general purpose of the signal is not - it's for ionosphere research: http://elpub.w

Whilst the meaning/intention of the voice messages may be "mysterious", the general purpose of the signal is not - it's for ionosphere research: http://elpub.wdcb.ru/journals/rjes/v10/2007ES000227/2.shtml http://elpub.wdcb.ru/journals/rjes/v10/2007ES000227/2.shtml Scroll down to "Doppler Radio Sounding of the Ionosphere", note the broadcast frequency. My guess is that the occasional voices are either a) someone else randomly broadcasting on the same frequency, b) some technical hitch at the broadcast station. Whatever the reason, 99.9999% it's going to be totally mundane.