Ah. I've only tried in on English (language) songs. It's handy if you are out in a cafe or otherwise, public area and want to identify a song.

Singing into the app would be cool. In my ex-company, there were folks working on getting it to work for Karaoke. So you can hum a song and it'll figure out the right track for you to sing with. I don't know if there's a commercialised product like this.