Suddenly, the vibrations seem good again. Pop maestro Brian Wilson, 53, brother Carl, 50, and cousin Mike Love, 54, are gathered round the microphone, just like old times, singing one of those unmistakable harmonies that so often lifted the Beach Boys to the top of the charts. But this is no oldies show. The three are actually working on a new song, happily crooning, ''Meet me somewhere out in Malibu!'' ''We're putting Carl's guitar on next,'' announces Brian Wilson, ''which will make it even more raucous. It might even fly away. It's good enough to totally fly out of the universe.'' Back on earth, in a Glendale, Calif., studio, the big surprise is that these guys are even speaking to one another. Just six weeks ago, Love won more than $5 million in a federal lawsuit against Brian Wilson, claiming that he was unfairly denied credit and royalties for ''California Girls,'' ''Fun Fun Fun,'' and more than 30 other Beach Boys hits he cowrote. And that doesn't take into account the darker side of the Beach Boys' history: Wilson's two-decade bout with mental illness, drug abuse, and various disputes with family, attorneys, and a former psychoanalyst, all of which have kept him mostly estranged from the band he once led. Ironically, says Love, it was the lawsuit that brought them back together, finally lifting the burden of unfinished business from their lives. Four weeks ago, he invited Wilson to his home in Lake Tahoe for their first serious songwriting session in two decades. ''We're back to square one,'' says Love, whose 6-year-old son, Brian, has added his voice to today's chorus. ''In a studio in a garage making demos towards a new crop of songs only it's 25 years later.'' Still, there's a small disagreement brewing. The song is tentatively slated for submission to a new syndicated TV spin-off called Baywatch Nights. But Wilson is so pleased with the results, he's reluctant to let it go for anything other than a Beach Boys album. ''We need this kind of a song,'' he insists. ''You can't throw away your ace.'' Wherever the song ends up, Love and Wilson hope that this is merely the first step in a renewed period of activity for the Beach Boys. Except for 1988's ''Kokomo,'' which was recorded without Wilson, the group hasn't had a No. 1 single since 1966's ''Good Vibrations.'' ''Only the voices will be the same,'' says Wilson of the new songs. ''The tracks are a little more hard-driving. I'm trying to get used to our new thing, and I think I will. It's so hard, you know. I feel like I'm on the spot, and I don't like that feeling.''