You're not incrementing "count" so you're just constantly overwriting the same row
foreach (var item in myMusicList)
{
ws.Cells[string.Format("A{0}", count)].Value = item.ID;
ws.Cells[string.Format("B{0}", count)].Value = item.Artist;
ws.Cells[string.Format("C{0}", count)].Value = item.Song;
ws.Cells[string.Format("D{0}", count)].Value = item.Genre;
ws.Cells[string.Format("E{0}", count)].Value = item.MusicDate;
count++;
}