Insert an MP4A file in the Middle of another MP4A file

Question

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.