At the time it was broadcasted in France, each broadcast of the whole series needed a new introductory music. This is why there are two different musics for the series. The first one is the plain "Ulysse 31" song, and the next one is "Ulysse revient" (aka Ulyses comes back) song.
The trivia item below may give away important plot points.
This was noticeable for being DiC's first series as an independent company, and the third DiC series ever produced. Cro et Bronto (1980) being the first, and Archibald the Magic Dog (1980) being the second.