Na stadionie słychać gwizdek sędziego.

Polish often uses na with places understood as venues, open areas, or event spaces, such as na stadionie (at the stadium), na boisku (on the field), na koncercie (at the concert).

So na stadionie is the normal idiomatic way to say at/in the stadium in this context.

W stadionie is possible in some situations, but it sounds more like you are emphasizing being physically inside the building structure, not just at the stadium as a venue.

Because na here expresses location, and with location it requires the locative case.

• stadion = base form
• na stadionie = in/at the stadium

This is a very common pattern:

• w domu
• na stole
• w lesie
• na stadionie

So the ending -ie shows that stadion is in the locative singular.

Słychać means something like:

• you can hear
• it can be heard
• one can hear
• there is the sound of

It is an impersonal form, so it does not change for person the way a normal verb like słyszeć does.

Compare:

• Słyszę gwizdek. = I hear a whistle.
• Słychać gwizdek. = A whistle can be heard.

In your sentence, słychać makes the statement general and descriptive, not tied to a specific person hearing it.

Because słychać is impersonal.

English often needs something like:

• you can hear...
• it is possible to hear...
• there is the sound of...

Polish does not need a subject here. The word słychać already carries that impersonal meaning.

So Na stadionie słychać gwizdek sędziego is a complete sentence even though no person is mentioned.

Here gwizdek is the thing being heard, so it is in the accusative case.

For a masculine inanimate singular noun, the accusative usually looks exactly the same as the nominative.

So:

• nominative: gwizdek
• accusative: gwizdek

That is why it looks unchanged.

If it were a different type of noun, the form might look different.

Sędziego is the genitive singular of sędzia.

It is used because Polish normally expresses X's Y by putting Y first and then X in the genitive:

• gwizdek sędziego = the referee's whistle
• literally: whistle of the referee

So sędziego answers the question whose whistle?

A useful thing to notice: sędzia ends in -a, but it refers to a male person and behaves as a masculine personal noun.

Usually, in this sentence, it is understood as the sound of the referee's whistle.

Literally, gwizdek is a whistle or a whistle blast, so context decides how concrete or sound-based it feels. With słychać, people will normally interpret it as something audible, so the meaning is naturally the referee's whistle being heard.

Słyszeć is a normal verb meaning to hear. It is conjugated for person:

• słyszę = I hear
• słyszysz = you hear
• słyszymy = we hear

Example:

• Słyszę gwizdek sędziego. = I hear the referee's whistle.

Słychać is different: it is impersonal and does not say who hears it.

• Na stadionie słychać gwizdek sędziego. = At the stadium, the referee's whistle can be heard.

So słychać is great for describing the general sound environment.

Polish word order is more flexible than English word order.

This sentence starts with Na stadionie to set the scene first:

• Na stadionie = location/topic
• słychać gwizdek sędziego = what can be heard there

That makes it sound natural and descriptive: first the place, then the sound.

You could rearrange it, but the emphasis would change. For example:

• Gwizdek sędziego słychać na stadionie.
  More emphasis on the referee's whistle.

The version you gave is a very natural neutral order.

Because Polish has no articles like English a and the.

Whether something is definite or indefinite is understood from context.

So:

• gwizdek sędziego can mean the referee's whistle
• or, in another context, possibly a referee's whistle

In this sentence, context makes the referee's whistle the most natural translation.

Yes, depending on style.

Besides You can hear the referee's whistle in the stadium, a natural English rendering could also be:

• There is the sound of the referee's whistle in the stadium.

That matches the impersonal, descriptive feel of słychać quite well.