ph.x does not yield acoustic modes with = 0 at q = 0 .

This may not be an error: the Acoustic Sum Rule (ASR) is never exactly verified, because the system is never exactly translationally invariant as it should be (see the discussion above). The calculated frequency of the acoustic mode is typically less than 10 cm^-1 , but in some cases it may be much higher, up to 100 cm^-1 . The ultimate test is to diagonalize the dynamical matrix with program dynmat.x, imposing the ASR. If you obtain an acoustic mode with a much smaller (let's say < 1cm^-1 ) with all other modes virtually unchanged, you can trust your results.