I now realize that another, much more significant, aspect of language was staring me in the face all that time, so obvious I simply overlooked it. Almost every single language in SubSaharan Africa is a tonal language, i.e., a language in which differences of meaning are determined by differences in pitch. Thus, if homo sapiens originated in Africa, as is now generally believed, the first language is very likely to have been tonal -- which tells us that, if HBP had any language at all, it too would have been tonal -- and even if they didn't have a fully formed, fully syntactic language (as I suspect they may not have), then whatever vocabulary they had would most likely have contained tonally differentiated phonemes. This observation ties in rather nicely, I must say, with earlier speculations of mine that the development of music may well have preceded, and influenced, the development of speech, because musical notes in and of themselves operate like tonal phonemes (see "Echoes of Our Forgotten Ancestors").
Regardless of whether a fully formed tonal language was part of HBC, such a language was almost certainly employed by HMP -- first, because they were in all likelihood SubSaharan Africans, but also because, unlike clicks, tonal languages are in fact widely found among their descendants. If we expect to conclude, however, that tonal languages were spread via the migration initiated by HMP, along the southern route, we have a problem. A very puzzling gap can be seen in the distribution of tonal languages along this route: