Let's be honest: The five-tone phrase sounds like a first-generation ringtone.

You'd think galaxy-hopping aliens would have invented technology sufficient enough to communicate by way of Misirlou, so why do they bother? Let's find out.

The Sound of Music

Spielberg enlisted John Williams to compose the now famous five-tonal phrase. The composer pushed for Spielberg to consider a seven-note sequence, but Spielberg insisted on five, believing anything longer would be too long for the simple "'sup?" the phrase was meant to imply. Williams created hundreds of potential phrases and at one point asked a mathematician "to calculate the number of five-note combinations [he] could potentially make from a 12-note scale." The mathematician came back with a number in the neighborhood of 134,000 combinations. (Source)

Since life is short and production deadlines shorter still, Williams and Spielberg decided to work with the couple hundred phrases already created. By simply listening to them one at a time, they eventually settled on the notes featured in the film. They are, in solfege: Re, Mi, Do, Do (one octave below the first), and So. Go watch The Sound of Music if you don't know what we're talking about here.

Galactic Guitar Hero

Williams's account of the events suggests Spielberg and he chose the five notes by ear and intuition, but what in his intuition led him to make the choice he did? Jeff Evans has a theory about that:

I think that the inhabitants of a distant galaxy would recognize and appreciate a melody formed of the major and minor scales, because they are both derived from universal acoustic principles: a vibrating string in another galaxy will have the same harmonic partials as it does here […], and among the first and most audible of those partials are Do, Do an octave higher, So, Mi, and Re. (Source)

In other words, music is truly the universal language. Human languages are wonderful and varied and beautiful, but they're uniquely human, based on a limited number of sounds we can produce thanks to our anatomy. Same goes for sign language but with different parts of our anatomy (e.g. hands, five fingers, location of face on body, etc).

The alien at the end of the film learns a little sign language from Lacombe, only because it has a five-fingered hand, proving our point. (It smiles, too—another universal message, but only because it has lips to smile with. Not all of them do.)

But if you pluck a guitar string, it makes a certain sound. If you give an alien that same guitar, it'll make the same sound. The physics determining sound (wavelength, frequency, the medium through which it travels) are universal constants, the same from here to Alpha Centauri. The ability to create instruments to manipulate sounds into particular patterns suggests a certain level of intelligence—pattern recognition, basic mathematical conception, environmental manipulation, tool building, and so on.

In other words, the aliens are giving us a brief IQ test. If we can respond, it proves we're smart enough to meet the minimum skills as a species. At the film's conclusion, humans greet their visitors with the musical phrase. As one of the scientists says, "It's the first day of school, gentlemen."

Looks like humanity has passed the entrance exam.