WARBLE | Probabilistic Model for Retrospective Event Detection | Machine Learning library

These files have an internal structure: header, blocks/frames, etc. and the simple presence of multiple headers within the concatenated file will mess up the expected result.

Take a look at the AAC file format structure, you'll see that it's not so simple.

Your best try should be to use FFMPEG, since it has a feature to concatenate media files without being forced to reencode data. It's a bit complex because FFMPEG's command line is quite complex and not always extremely intuitive, but it should works as long as all AAC files uses the same encoding and characteristics. Otherwise, you'll need to re-encode them - but it can be done automatically, too. Check this web research to get some base informations.

Otherwise, you may use the base libraries used by FFMPEG, for example libavcodec (available at ffmpeg.org), Fraunhofer FDK AAC, etc. but you'll have way, way more work to do and, finally, you'll do exactly what FFMPEG already do, since it relies on these libraries. Other AAC libraries won't be really easier to use.

Obviously, you can also "embed" FFMPEG within your application, call tools like ffprobe to analyze files and call ffmpeg executable automatically, as a child process.

CAUTION: Take a GREAT care about licensing if you plan to distribute your program. FFMPEG licensing is really not simple, most of the time it's distributed as sources to avoid vicious cases.