No sabes lo rápido que se me pasa el cansancio cuando escucho música.

Because No sabes... is often used idiomatically as an intensifier, not as a literal statement about what the other person knows.

Here it works like:

• You have no idea how...
• You don’t know how...
• You wouldn’t believe how...

So No sabes lo rápido que... is a very natural way to emphasize the next idea.

Lo rápido que means how fast or how quickly.

This is a common Spanish pattern:

• lo + adjective/adverb + que

It introduces an embedded exclamation, especially after verbs like:

• saber → No sabes lo difícil que fue
• ver → Ya ves lo caro que es
• imaginar → No imaginas lo lejos que está

So in your sentence:

• lo rápido que se me pasa el cansancio
  = how quickly my tiredness goes away

The lo here is the neuter lo, not the.

Because after lo, Spanish normally uses the adjective form, even when English would prefer an adverb.

So:

• lo rápido que... = how quickly...
• not lo rápidamente que...

In this construction, rápido is functioning in a way that English would often translate with quickly, but Spanish naturally uses rápido.

This is very common:

• lo fácil que es = how easy it is
• lo bien que canta = how well he/she sings
• lo rápido que corre = how fast he/she runs

Here pasarse means something like:

• to go away
• to wear off
• to pass

So se me pasa el cansancio means:

• my tiredness goes away
• more literally, the tiredness passes for me

It is not the basic pasar meaning to pass in the sense of movement, like the bus passes by. It is a pronominal use: pasarse.

In this sentence, el cansancio is what disappears.

They do different jobs.

• se is part of the pronominal verb pasarse
• me shows the person affected: to me / on me / for me

So:

• se pasa el cansancio = the tiredness goes away
• se me pasa el cansancio = the tiredness goes away for me / my tiredness goes away

The me makes clear whose tiredness is being talked about.

Because Spanish unstressed pronouns follow a fixed order. When se appears with another object pronoun, se comes first.

So:

• se me
• se te
• se le
• se nos
• se os
• se les

Me se pasa is not correct.

Spanish often uses the definite article instead of a possessive when the owner is already clear from context, especially with:

• body parts
• clothing
• physical states
• feelings

Here, me already tells you whose tiredness it is, so el cansancio sounds natural.

Compare:

• Me duele la cabeza = My head hurts
• Se me pasa el cansancio = My tiredness goes away

Using mi cansancio would not be impossible, but it would be less natural in this context.

The subject is el cansancio.

You can tell because the verb is singular:

• se me pasa el cansancio

If the subject were plural, the verb would also be plural:

• se me pasan las ganas
• se me pasan los nervios

So even though el cansancio comes after the verb, it is still the subject.

Yes. Spanish word order is more flexible than English.

The original version:

• se me pasa el cansancio

is very natural.

But you could also say:

• el cansancio se me pasa
• no sabes lo rápido que el cansancio se me pasa cuando escucho música

Those alternatives are possible, but the original sounds more natural and fluid. Spanish often places the subject after the verb in sentences like this.

Because the speaker is talking about something habitual or generally true.

• cuando escucho música = when I listen to music

This refers to a repeated situation, so the present indicative is normal.

Spanish often uses the subjunctive after cuando only when the event is in the future or not yet realized:

• Cuando escuche música, se me pasará el cansancio
  = When I listen to music, my tiredness will go away

But in your sentence, the meaning is general: this happens whenever I listen to music.

Because escuchar música usually means to listen to music in a general sense.

Spanish often omits the article with uncountable or general nouns in expressions like this:

• escuchar música
• tener paciencia
• beber agua
• comprar pan

If you added an article, it would usually sound more specific:

• escucho la música = I’m listening to the music / that music

But here the meaning is general, so música without an article is the natural choice.

They are related, but not identical.

• Qué rápido se me pasa el cansancio... is a direct exclamation: How quickly my tiredness goes away!
• No sabes lo rápido que se me pasa el cansancio... is an embedded exclamation inside a larger sentence: You have no idea how quickly my tiredness goes away...

So both express strong feeling, but No sabes lo rápido que... is more conversational and directed at someone.

It is the present tense, and in Spanish that can often cover both ideas depending on context.

Here, because of cuando, it is understood as a habitual action:

• cuando escucho música = when I listen to music

It does not usually mean when I am listening to music right now in this sentence. The whole sentence describes a general pattern, not a one-time event.