The sixteen strings on this instrument all are tuned to the same note but for a few that are tuned an octave higher. They’re positioned very close together, making it natural to strum across all sixteen rather than plucking them individually. To control the pitch, there’s a keyboard-like arrangement of thirty spring-mounted bars positioned over the strings so that each one, when pressed down, stops the strings at the correct point to yield the intended pitch. You can also play without strumming simply by sharply pressing the keys (the bars excite the strings when they hit). With so many strings all playing the same note, the tone is richly chorused. Alt-alt is closely related to the instrument called megalyra-alt, but different approaches to tuning on the two instruments means that the way they sound, and the ways they’re typically used musically, are quite different.
In the audio clip, Alt-alt is double tracked and dueting with itself. A video of this instrument can be found among the demonstrations in this youtube playlist.