Some other Android options include the free online video converter at amoyshare.com
https://www.amoyshare.com/video-to-mp3Put in the song title and artist in their search bar and their web site will locate the youtube video for it and let you download as an mp3 music file. You can either download on a pc/laptop or directly on your Android. They do limit you to 10 per day, but that works reasonably well in a couple days to get all you need for the app.
A little more expensive would be buying the songs individually from Amazon, download on a pc and copy to your device with usb cable. You can also use the Amazon music app to purchase and download on your Android. They charge like 1.19 per song.
A little less expensive would be buying the songs individually from mp3million.com , m.mp3va.com or similar discount mp3 sites. They charge around .10 per song and the files can be directly downloaded on your Android.