This app was mentioned in 3 comments, with an average of 1.33 upvotes
I use this.
Sounds like this might be able to do the job:
https://play.google.com/store/apps/details?id=igost.music.mp3cutter
I've had good results with this app in the past.
Top three Google results for "combine audio files android":
You didn't specify what types of audio files, or whether or not you wanted to do any selective editing, but for just combining files, one of those ought to work.