< proving that our evolutionary cousins could speak is pretty much impossible, given the absence of contemporary sound recording equipment
Well: a load of jokes, but no serious comments.
Consider the findings that the tool-making skills of the Neanderthals were not too inferior to those of homo sapiens of the same period and that Neanderthals buried their dead, with tributes of flowers thrown into the graves (totally unlike the behaviour of the present-day great apes, who leave their dead to rot where they fall, and move on).
The findings indicate a culture, and this is not possible without language. In particular, the burial of the dead indicates that the Neanderthals were capable of comprehending death, and thinking beyond it: a facility not possessed by any animal other than modern homo sapiens (i.e., us).
The more I hear about Neanderthals, the more I admire them.