The only advice I can give to you at the moment, is check that the frequency of the files is a valid one... mostly this tends to be either 8khz, 22khz (22050) or 44.1khz.
That is probably it - the original poster is using Microsoft Anna on Windows 7, which is a 16KHz-only voice.
The first thing to do would be to make sure that the old PhatNoise Music Manager was actually able to create non-corrupt speech files. Do this by connecting the DMS to the PC and browse to the TTS subdirectory of the PHTDATA partition. Look for .wav files in there. Try to play one in Windows. If it plays properly, then you almost certainly have a bit rate problem.
If it is a bit rate problem, read the
PhatVoice manual (
here) for the steps needed to convert all of the speech files to 22.05KHz.
The problem arises because the PhatBox is
not a generalized media player - it is actually quite picky about what file formats it will play. Generally, it wants audio that is encoded at a 44.1KHz (CD) bit rate. 22.05 is easy because it can be doubled. PhatNoise added special compensation for 8KHz because that's what the voice software they were using (an obsolete version of AT&T Natural Voices) generated. There is
no support in the box for 16KHz, 48KHz, or other bit rates. This is also true for the other file formats like MP3.