AAC Audiobook Creator

That depends.

A player software (usually 32 bit) must be able to address each sample of an audiobook file. Thus, there are exactly 2³¹ = 2,147,483,648 samples possible because one bit is lost for the sign.

With the usual sampling rate of 44,100 Hz the maximum length of an audiobook is therefore 2147483648 / 44100 Hz = 48695.8 seconds = 811.6 minutes = 13.52 hours.

With a sampling rate of 22,050 Hz the maximum length results to about 27 hours.

In case of using multiple input files, both, sampling rate (e.g. 44.1 kHz or 22.05 kHz) as well as the number of channels (mono, stereo) has to be identical for all files. Otherwise the new audiobook is not accepted by iTunes or the iPod. Before converting the AAC files this has to be taken into account.

The preferred bit rate (e.g. 128 kBit/s) however may be different.

since Version 2.0:
 
iTunes refuses to accept an audiobook as well, when the underlying AAC files (*.m4a) have been produced with different encoding tools (e.g., iTunes and "neroAacEnc.exe"). In this case all files have to be encoded again from their MP3 or WAV origins by using only one encoder.

The error message is produced under certain circumstances by the operating system during the start of the command line program "SSA.exe".

To avoid this message permanently "SSA.exe" has to be executed once with administrator privileges. For this purpose either start the Audiobook Creator with administrator privileges and compile an audiobook (SSA.exe will be executed during this task) or start "SSA.exe" directly in a command line that has been started with administrator privileges before (right mouse button in Windows Explorer "Start as administrator", standard path: "C:\Program files (x86)\AAC Audiobook Creator\Tools\SSA\SSA.exe).