Whistled sibilants occur in speech pathology and may be caused by dental prostheses or orthodontics. However, they also occur phonemically in several southern Bantu languages, the best known being Shona. These have been variously describedas labialized but not velarized, retroflex, etc., but none of these articulations are required for the sounds.[1] Using the Extended IPA, Shona sv and zv may be transcribed [s͎] and [z͎]. Other transcriptions seen include purely labial [s̫] and [z̫] (Ladefoged and Maddieson 1996) and labially co-articulated [s͡ɸ] and [z͡β].
Thanks for the info! Interesting.