It sounds like you're trying to edit a DRM protected file. Have a read of: www.nch.com.au/acm/support.html#OMA
P. OMA / OMG / ATP files do not load properly / Only the first 1.01 minutes of my OMA / OMG / ATP file gets converted
We only support the older-style ATRAC .wav format, we do NOT support the OMA/OMG/ATP formats, as these have in-built Digital Rights Management (DRM) protection. You need to contact Sony to ask how to convert these files to other formats.