Alle hører fløyten når dommeren blåser.

In this sentence we talk about a specific whistle—the referee’s. Norwegian marks definiteness by adding a suffix to the noun rather than using a separate article.
• fløyte = “a whistle” (indefinite)
• fløyten = “the whistle” (definite)
So Alle hører fløyten literally means “Everyone hears the whistle.” Saying en fløyte would mean “a whistle,” which changes the nuance to any whistle rather than the referee’s.

In Bokmål, fløyte is a common‐gender noun (historically feminine). Common‐gender nouns can take either the masculine‐style ending -en or the feminine‐style ending -a in the definite singular.
• fløyten = definite with masculine‐style suffix
• fløyta = definite with feminine‐style suffix
Both are correct, though fløyten is more widespread in everyday Bokmål speech.

We refer to a specific person—the referee in the game—so we use the definite form.
• dommer = “a referee” (indefinite)
• dommeren = “the referee” (definite)
Using dommeren makes it clear we mean that particular referee who’s about to blow the whistle.

The verb is å blåse (“to blow”). Its basic conjugation is:
• Present: blåser
• Past: blåste
• Perfect: har blåst
So dommeren blåser = “the referee blows,” dommeren blåste = “the referee blew,” etc.

da is generally used for a single, specific moment or event in the past (“when I arrived, he left”). når is used for:
• Repeated or habitual events (“when the referee blows the whistle, everyone hears it”)
• Future situations (“when it rains tomorrow…”)
Since this sentence describes what happens whenever the referee blows, når is correct.

• høre = “to hear” (passively perceive a sound)
• lytte (til) = “to listen (to)” (actively focus on a sound)
You can indeed say Alle lytter til fløyten to emphasize that everyone is paying attention to the whistle, but Alle hører fløyten simply states that everybody hears it.