Insert an MP4A file in the Middle of another MP4A file

I have a project requirement where I have to record two audio files, and them insert the second file in the middle of the first file. It is a Cordova app, currenlty I use "cordova-plugin-media" to do the recording, by default on Android the file extension is MP4a. after recording I upload both files to the server. on the server I use NAudio to split the first file and then merje the 3 files together also using NAudio. The code is working, the problem is that the seek on the Mobile is not working and I believe it is because the RIFF files is corrupted when the spliting and the concatinating was performed. So I am looking for a way to properly do these steps and have a valid RIFF file or a 3rd party .Net library that can perfrom these tasks.

Thank you for all your input in advance.

If your resulting concatenated file has the correct audio data, but its container has a problem that prevents it from working correctly (such as doesn't seek), you could try a simple multimedia conversion on this file in order to build a valid container from that audio data..

One way you could do this is through the DirectShow-based Convert Control from the LEADTOOLS Multimedia toolkit:

https://www.leadtools.com/sdk/engine/multimedia

Using the .NET edition of this control, you can implement something like the following code:

ConvertCtrl1.SourceFile = "Input_File.mp4";
ConvertCtrl1.TargetFile = "Output_File.mp4";
ConvertCtrl1.AudioCompressors.AAC.Selected = true;
ConvertCtrl1.TargetFormat = TargetFormatType.ISO;

ConvertCtrl1.StartConvert(); // Note: Don't abort the process until conversion is done