★★★★★ The problem is found due to the corruption of the files. I don't know why it always failed to download them to my phone while it didn't to PC, but PC version showed the message that the file is corrupted and files copied from my PC was played correctly. Thank Matthieu Leschemelle for the helpful reply. Another little problem is, the notes-denoting line following them is sometimes delayed or placed on no notes, not a serious problem but slightly confusing. Overall, it's great app. // It doesn't open .tef files for banjo and only can play .midi files for banjo without tablature. I wish it will support banjo as PC version do.