Welcome to Android Central! Music player apps usually depend on a music file's embedded ID3 tag for organization (i.e., by album, artist, etc.). So even though you have a whole bunch of songs in a physical directory together, the player app will probably just play the one song because it thinks you want to stop after that song's album (of which you only have one song) is done. If you want to play all the songs in a physical storage directory, then you either have to create a playlist of all of those songs, or use a music player app that can play by folder, like MortPlayer Music or PowerAmp.