According to
this TCAS software 7.0 improvement analysis, The audio warning had been abandoned with the (previous) version 6.04a, see page 15. Version 7.0 is available approximately from 2000, so there may be older hulls with not updated software. Although the Eurocontrol borchure linked at the end of my previous post, clearly states that v 6.04a is not opeartionally compatible with RVSM.