YOU CAN'T go far discussing pop music in categories so broad as regional "sounds." For instance, what is the San Francisco sound - Creedence Clearwater Revival, Jefferson Airplane or Sly and the Family Stone? Or the Nashville sound - Bob Dylan, Judy Collins or Joan Baez (they all record there now)?

There is also the famous Motown sound, perfected into a hit-making formula so refined that it eventually almost strangled in its own sameness.

Then there is the Canadian sound, which is where I was headed all this time. Here we get back to the impossibility previously mentioned; how can you take a group of artists from a huge country and say they all sound alike?

Well, you can't. And yet ... listen to Zal Yanovsky of the Lovin' Spoonful, and Ian and Sylvia Tyson, and The Band, originally a Canadian group called Levon and the Hawks. Some are transplanted, like Neil Young, who left the Buffalo Springfield and new solos, or Joni Mitchell, who now lives in Southern California, or Leonard Cohen, who, I suppose, still spends most of the year on an island in the Mediterranean. Some have stayed home, like Gordon Lightfoot, the Collectors and the Guess Who, who are just now surfacing.

But when you listen to them all at a single sitting, something strikes you about most of their widely varied music. It is a feeling of grandeur first of all, a bigness of perspective, whether it is in wide-open songs or intricate, detailed descriptive ballads. Another thing is a certain crispness, a cleanness of phrasing that doesn't seem to have been influenced by the mumble-rock of the U.S. '50s. Evidentally, the idealistic nature-loving folk song movement has made a deeper impression up north. But for whatever reasons, the Canadian sound is beginning to make itself heard in the American pop scene.

Leonard Cohen writes poetry and songs. He is what I would call a middle-depth poet. His poems can be looked at closely, but not very closely. The songs are better because no one would think of looking extra close at a song - well, almost no one.

His second album, "Songs From a Room" (Columbia), has 10 excellent songs that can be listened to closely. Actually you must do this because they are so subtle that otherwise they will sound the same.

His voice is flattered by calling it a monotone; it cracks when he tries to sing louder than conversational level. And yet this helps the listener concentrate on the beauty of the songs by understating them.

You should play "Songs From a Room" twice before listening to it. Except for "Story of Isaac," a masterpiece at first hearing, the songs have to grow on you. But once you have listened closely, the album will never leave you and you will realize that it is almost perfection in its own strange art. The lyrics are middle-depth, similar to those in "Songs of Leonard Cohen" (Columbia), the first album. They are mystical, haunting and wonderful.

The album owes a lot to someone else, who might be called a genius in his field, Bob Johnston. Johnston, who also supervises Bob Dylan's recordings, is probably the greatest record producer around.

JONI MITCHELL has the problem-asset that Joan Baez Harris has - a voice that jumps from soft-sweet to loud-funky, skipping the registers in between. And like Joan, she makes good use of this attribute, notably in her new album "Clouds" (Reprise).

Mrs. Harris, it must be admitted, has a better voice. However, few women can approach Joni Mitchell in songwriting. The album's name is one of the two titles under which her one hit has been recorded (the other is "Both Sides Now").

But hit or not, her 10 songs here are enchanting exercises in femininity, with gossamer melodies and stirring lyrics.

And the question of voice never enters into it, because Miss Mitchell's voice has the qualities that perfectly realize her songs. I even like her version of "Both Sides Now" better than Judy Collins'.

GORDON LIGHTFOOT had a brief burst of popularity during the big folk-music fad. "Early Morning Rain" and (that's what you get) "For Lovin Me" were widely sung and Marty Robbins made a country hit out of "Ribbon of Darkness."

But Lightfoot has not disappeared while being ignored by the mass audiences south of his border. He has four excellent albums out, "Lightfoot," "The Way I Feel," "Did She Mention My Name" and the recent "Back Here on Earth" (all United Artists). And his songwriting and singing has matured and mellowed, which has meant improvement. When he finally comes up from underground, he'll do so with a bang, I predict.

The Collectors are also the object of general ignorance down under, but their music for "Grass and Wild Strawberries" (W7), a play written by George Ryga, a Canadian, shows why they're big in Canada and have been commissioned to write the music for the Canadian exhibit at the Japanese World's Fair. They are rockers and the album is something special.