Board Thread:General Discussion/@comment-25902107-20150305190234/@comment-26394058-20160131035143

Foxes usually aren't around humans when they speak, but given their body shape and throat size, they make a midrange-high pitched sound, from barking to growling. Since that is all speculative, it is possible that they make a sound above or below human hearing.

I'm not a professional, that was all logic and guesswork