What happens if someone plays an instrument without knowing how to bring out the right tone at the right time?
If an instrument is played without the skill to bring out the right tone at the right time within the score, the result is a bunch of noise. There is no distinction in sound, no melody or harmony, and no one can tell what is being piped or harped. Paul says the same principle applies.
Les explains that unless someone picks up an instrument knowing how to play it and how to bring out the right tone at the right time within the score, all you have is noise. He compares this to a symphony orchestra before the curtain rises: the musicians are tuning, and it sounds like gibberish, with no melody, harmony, or anything worth listening to. You can hardly wait for them to be ready so the curtain can go up and you can hear music. Paul is saying the same thing about instruments like the pipe or harp: except they give a distinction in the sound—the right note at the right place and right timing—no one knows what is piped or harped or what melody is being played. The next verse adds that if the trumpet gives an uncertain sound, no one prepares for battle; the Romans used the trumpet for battle commands.
And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped?