I love that tune, and it is indeed Gardel’s “Por Una Cabeza.” The song’s on the soundtrack albums of neither *Scent of a Woman * nor True Lies, IIRC, but I found it on iTunes by The Tango Project. The performance is either the exact same one used in the movies, or damned close. Check it out.
I’m surprised you couldn’t find anything on IMDb.com. I posted to a True Lies thread about the tango there awhile back; it’s a recurring question.