Pop-Rock Electric Light Orchestra: ''Secret Messages.'' (CBS/Jet Records QZ 38490.) We've heard all this before from Jeff Lynne and his Electric Light Orchestra - though at times the music sounded a bit more focused. The introductory noises to the title song ''Secret Messages'' sound somewhat like an abbreviated version of the prelude to ''Fire on High'' on ELO's ''Face The Music'' effort. The '50s rock bit they do on ''Four Little Diamonds'' has surfaced a number of other times. And the distinct sound of synthesizers working overtime to sound vaguely like a symphony orchestra is the same. That said, ''Secret Messages'' is an inoffensive, companionable addition to the ELO collection, though the flip side is much better than the first side - which gets bogged down in heavy-handed synthesizers. On Side 2, the music emerges triumphant, with tighter arrangements and excellent pop licks. The before-mentioned ''Four Little Diamonds'' song is a spiffy rocker, and the ''Stranger'' song that follows a bit more introspective, though still in keeping with the vibrant pulse of the LP.