Perhaps if I had played the ukulele?

IF anyone intends to compose music, it would seem sensible to gain some knowledge of it and to play a musical instrument - at least a ukelele. Years ago I played the mandolin a little, by ear. Now all I play is a hi-fi, or stereo as many prefer to call it, although the term ''high fidelity'' refers to the quality of the sound, not to the number of the speakers. Still, if you enjoy composing, why allow a lack of musical knowledge to deter you? That, as the song says in ''Show Boat,'' is just a technicality. I have read of successful composers who could not read music when they began and could play the piano with only one or two fingers.

I did study the piano for a brief period, and even learned a piece called ''The Answer.'' Not by Chopin. But, alas, I was more interested in the fourth ''B'' than in the first three - Bach, Beethoven, and Brahms. In case you're not familiar with the fourth, it is baseball.

Once I composed the lyrics and music for a song I called ''It Only Happens in the Movies.'' All I could do was whistle it, so I asked a musical friend of my mother's to get it down on paper. I thought it had a sprightly air, but eventually the lyrics sounded dated. Later I composed a waltz for which a friend , a newspaperwoman, wrote the lyrics. I took it to a publisher and asked him to tell me, straight off, whether it was bad. He played it and said: ''No, it's not bad. You might have sold it some years back, but the style is rather dated.''

My next musical adventure, involving a march, sprang from a sentimental poem I wrote in grammar school. It was about a pretty girl who often wore a blue middy. Years after, I came across the poem and as I read the first lines, ''She's the girl in the middy of blue,'' the beat of a march came to mind. I'll spare you the saccharine sentiments that followed.

Without being immodest, I think I can say I have written better poetry since. When a voice teacher was visiting in our home I told him I'd like to have the music written down. He advised me to see a musical arranger who worked regularly for two movie magnates. When I suggested that such a musician wouldn't want to bother with a project as unimportant as mine, he assured me that the arranger had done work for him, that he was a fine gentleman and would be glad to accommodate me.

I told the arranger I knew it was a bit silly to attempt to compose when I could only whistle the tune. But if he thought it was a foolish idea, he was too polite to say so. After I whistled a part of the march he commented, ''Yes, there's an idea there.'' Later, when I picked up the music, he said, ''You didn't have a trio, so I wrote one.'' All for five dollars! He was indeed a good and kind man. I was so ignorant then that I didn't even know what a trio was, unless you were referring to that famous Chicago Cubs baseball trio, ''Tinker to Evers to Chance.''

In case you're not a musician, I'll explain. As you know, a march begins in a vigorous manner, bang, bang, bang. Then it changes to a quieter, more harmonious melody before returning to the vigorous style. The quieter part is called the trio because, originally, it was played by three instruments.

''The Music Lovers' Encyclopedia'' (Doubleday) defines ''trio'' in part, thus: ''In the dance-form, the contrastingly quiet or lyrical second division,'' and this applies to the march also. The encyclopedia continues, ''The word should now be laid aside as meaningless and confusing and the phrase 'second part' or 'second subject' used instead.''

When my daughter was small she tore my only copy into four or five pieces. Perhaps she was just exercising her critical prerogative. I pasted the pieces together and took them to another musical arranger, the first one having passed on. While I was there, a man who hoped to sell a song came in. As we chatted the arranger remarked, with a laugh, ''You don't have to be crazy to be in the music business, but it helps.''

When I picked up my new copy from the arranger, who was also a friendly person, he said, ''I changed the time from 2/4 to 4/4. That will give it more'' - he gestured to show what he meant, which, translated, might mean zing.

It sounds odd, especially since my first arranger was a fine musician, but the trio he wrote causes me some misgivings. It seems a bit complex. Most trios , whether by Sousa, Goldman, or whomever, can be whistled without much difficulty; the one in my march cannot be. Someday I plan to write another trio. That will be progress, won't it?

Does anyone really compose music? Isn't it rather that the music is already there and we just listen for it?