We are building a exercise app using ionic framework, which need to play multiple audio files in sequence with specific interval between each audio file. We got this working
You could create a cordova plugin that manage the files concat in the javascript layer but really join them in the native layer.
Other interesting question about audio merging (only in Android):
Android concat 2 audio (wav or mp3) files
How to merge two mp3 files into one (combine/join)
Concatenate two audio files and play resulting file