make sure that the audio is in an actual mp3 codec, sometimes the file name ends with .mp3 but is in another uned codec that osu! doesn't . Also make sure that the file name contains
only standard unicode characters (so no accents or non-english characters)
Also, if you could the beatmap on some file hosting (like Mediafire, Mega, Dropbox ecc..) that would help us understand what's going on