That particular error can come up in Audiobook Builder for a few different reasons, so we'd like to gather some more info about what exactly's going on behind-the-scenes. Specifically, seeing a System Information report generated just after the issue occurs would be quite helpful since this report will include logged background processes on your Mac as well as some general information about your unique setup there. So, get Audiobook Builder to present the same error message and then just after launch System Information.app from your Applications/Utilities folder. Then, use the Save option under the File menu and attach the resulting report file to an email to
support@splasm.com.
Also, if you could tell us more about the file(s) being used, that would be helpful. Like, what's the file's duration, size, and sample rate? To determine these things, select the file in the Finder and choose 'Get Info' from the File menu.