Apparently, there was a old movie that was translated to Spanish and ended up with the name "El Danubio Azul". When I google, all I can find is a musical piece from Germany, who's translation is also "El Danubio Azul". The original German waltz, titled "An der schönen blauen Donau", was composed by Johann Strauss II, back in 1867.
The musical piece was also used in the 1961 movie "2001: A Space Odyssey". But I'm pretty that is NOT the movie my mom is talking about. :\
PS: Mom said it was NOT a silent movie, but it was black and white, NOT cartoons. IMDB says it should be this: The Blue Danube - 1932 ... still not an inch closer to procure a copy. Should something so old be public domain?