Music Editor is a multi-purpose audio editor for Android. We can use it to trim, merge, convert, mix, compress, tag, split, reverse, mute audios and music on our mobile phones. Find out more details and download the free music and audio editor app here. Today, we will use this lightweight app as audio mixer to overlay multiple audio files on Android mobile. It is easy and straightforward.
Launch the free audio mixer for Android on the phone. You will see its home screen like this.

You will find its main functions on the home screen, such as Trim Audio, Merge audio, Audio converter, Video to Audio. Tap More to reveal more functions.

Select Mix audio from above screen. Then add audios, recording and/or music songs from Android to the music editor. Proceed to the next screen, you will have the options to adjust the volumes of each sound tracks, select output duration according to the shortest or longest audio file in the list.

Touch the Play button at the bottom for preview. When you are satisfied with the result, tap the Save button at the top right corner. All selected audios, music, sound recordings will be mixed into one MP3 file.
You can find the mixed sound, recording or audio tracks from My Creations > Mixed in the music editor, or access all mixed audio tracks in MP3 format from Internal storage > DCIM > Music Editor > Mix using a file explorer app on your phone.