Mieli rauhoittuu, kun kuuntelen musiikkia.

Here mieli means mind, state of mind, or inner feeling. It is not the physical brain; it is more about your mental or emotional state.

So in this sentence, mieli is the thing that calms down.

Finnish has no articles, so there is no separate word for the or a/an.

That means mieli can mean things like:

• a mind
• the mind
• just mind in a general sense

The exact meaning comes from context.

If you want to say my mind explicitly in Finnish, you would usually say mieleni or minun mieleni.

• mieleni = my mind
• minun mieleni = my mind with extra emphasis

The version Mieli rauhoittuu, kun kuuntelen musiikkia sounds a bit more general, almost like a general observation: the mind calms down when I listen to music. Because the second clause has kuuntelen (I listen), many learners will still understand it as the speaker’s own experience.

A more clearly personal version would be:

• Mieleni rauhoittuu, kun kuuntelen musiikkia.

Rauhoittuu comes from the verb rauhoittua, which means to calm down or to become calm.

The ending -uu here is the 3rd person singular present form, because the subject is mieli, which is singular:

• mieli rauhoittuu = the mind calms down

So even though the second clause has I listen, the first clause has a different subject: mieli.

Not exactly. Finnish usually does not use reflexive pronouns the same way English sometimes does.

Here rauhoittua is an intransitive verb: it means that something becomes calm.

Compare:

• rauhoittua = to calm down
• rauhoittaa = to calm someone/something down

So:

• Mieli rauhoittuu = The mind calms down
• Musiikki rauhoittaa mieltä = Music calms the mind

In Finnish, the verb ending usually tells you who the subject is. So kuuntelen already means I listen or I am listening.

Because of that, minä is often left out unless you want emphasis or contrast.

• kuuntelen = I listen
• minä kuuntelen = I listen with stronger emphasis

This is very normal in Finnish.

Here kun means when. It introduces a subordinate clause:

• kun kuuntelen musiikkia = when I listen to music

In other contexts, kun can also mean because, but in this sentence the meaning is clearly when.

Musiikkia is the partitive singular of musiikki.

Finnish often uses the partitive with objects when the action is:

• ongoing
• incomplete
• general
• aimed at an uncountable substance or mass

Listening to music is usually seen as an ongoing activity, not something with a clear finished result. That is why kuunnella often takes the partitive in this kind of sentence:

• kuuntelen musiikkia = I listen to music

If you are listening to a whole specific item, Finnish may use a total object instead:

• Kuuntelen kappaleen. = I listen to the song / I’m listening through the whole song.
• Kuuntelen albumin. = I listen to the album.

But for music in general, musiikkia is the natural form.

Finnish present tense can cover both meanings. Context decides.

So kuuntelen can mean:

• I am listening
• I listen
• I do listen

In this sentence, the most natural meaning is general/habitual:

• when I listen to music
• whenever I listen to music

It sounds more like a general fact than something happening only right now.

In Finnish, subordinate clauses are normally separated by a comma.

So in:

• Mieli rauhoittuu, kun kuuntelen musiikkia.

the comma marks the boundary between:

• the main clause: Mieli rauhoittuu
• the subordinate clause: kun kuuntelen musiikkia

This is standard Finnish punctuation.

Yes. You can also put the kun clause first:

• Kun kuuntelen musiikkia, mieli rauhoittuu.

This means the same thing. The difference is mainly in focus and rhythm:

• Mieli rauhoittuu, kun kuuntelen musiikkia.
  starts with the result
• Kun kuuntelen musiikkia, mieli rauhoittuu.
  starts with the condition/time clause

Both are natural.