I tried to use dbpoweramp but it kept failing. I tried a few others but they all failed too
Turns out the itunes rip from CD to ALAc was poo. There were bits missing in the header info which itunes was happy not having but all the converters needed.
It meant I couldnt convert my collection. I've had to re-rip the CDs into flac using dbpoweramp.
It was an old versions of itunes either, this included the latest version with some new CDs I had. Also, it wassnt every track either some times all tracks were fine, sometimes one or two tracks wouldnt convert. they played fine in itunes. There were problems streaming to roku but I assumed it was a naff wifi connection but now its probably the alac to wav conversion that was failing.