One major cause of -43 errors that we uncovered in preparing Audiobook Builder 1.1.3, and that may apply in your case, is that QuickTime allows us to get away with setting up an export that exceeds its 32-bit limitation (approximately 2.1 billion audio samples) but will fail when asked to go through with the export to MPEG-4/AAC. It works fine if you simply flatten the data to an MOV - but that's not how we roll. So, we had Audiobook Builder setting up each export to fit within the maximum length per the output audio sample frequency. If the output frequency is lower than the input source material's the maximum Target Length can exceed QuickTime's capabilities and the odds of an error -43 popping up, while not guaranteed, are rather good. In 1.1.2 you can usually work around this by setting the Target Length to something lower, trying to get it at or below the maximum allowed under QuickTime's 32-bit limit: 13 hours for 44.1kHz, 18 hours for 32kHz, 27 hours for 22.05kHz, and so on.
1.1.3 does a better job of handling this by examining the sample rate of all source material to be sure what goes into a single export doesn't exceed 2.1 billion samples. We're still testing 1.1.3 and you're welcome to test along with us. Drop us a line at
feedback@splasm.com and we'll send the details. If the current beta doesn't resolve the -43 errors you're observing we'd like to know so we can zap whatever else may be causing them.