first of all you are using old drivers, latest one seems to be 4.7.3. There is still bug waiting to be resolved. Also check format of audio file, it should be 64 Kbps, 8000 Hz, 1 channel, 0x6 = CCITT A-law coder.
there is a bug in ivm software, I tested new version, received from support and it does not skip beginning of message, but sometimes skips middle of message. It happens about once in a five calls.
I found in Axon, IVM and Carousel button called toolbox, which opens a link to components page http://www.nch.com.au/components/index.html, but there is no such component to download 'tbsetup'. Anybody knows where to find it?
Nothing helps, even high priority for IVM process. The problem is with communication between IVM and Synway SHT-8B/PCI I believe, because VOIP calls do not give audio skips at all.