there are recording digital audio software for android that are referred to as digital audio workstations or DAW's in effect youd be merging two songs together to create one file, Its a fairly intuitive process of opening the DAW and loading the song, scrubbing to the end and recording your voice via the devices mic, then loading the other song afterwards and doing your voice at the end of it on ONE track. I am not sure if you can just place the record cursor at the end like most daws and hit record but it should be possible. then you save the file or export it in the audio format you want, BUT Never a higher resolution than the original file E.G. if your file is an MP3 @ 320 mbps etc. Keep it there or you can reduce it for filesize mayyers , going upwards could give you audio artifacts.