Learn Hardcore Japanese: piano no oto ga ookii node, watasi ha mimi wo husagimasu. - The piano sound is loud, so I cover my ears.

が marks the grammatical subject in a descriptive clause with an adjective.
Using は here would make 音 the topic and could imply contrast (“As for the sound, it’s loud, but…”).
を marks direct objects, which doesn’t fit when you’re stating a property (being loud).

What’s the difference between ので and から, and why is ので used here?

Both mean “because,” but:
• ので is softer and often more polite/neutral, used for explanations or justifications.
• から is more direct and common in casual speech.
In this sentence, ので gently explains the reason for covering one’s ears.

Why can we link 大きい directly with ので without adding だ?

大きい is an い-adjective, so you attach ので straight to its plain form. If it were a な-adjective or noun, you would use なので (e.g. 静かなので).

What does 耳をふさぎます literally mean, and what is ふさぎます?

耳をふさぎます literally means “cover (one’s) ears.”
ふさぎます is the polite non-past form of the transitive verb ふさぐ, which means “to block,” “to close off,” or “to plug.”

Why is 私 followed by は in the second clause?

After giving the reason with ので, the speaker shifts to their action. は marks 私 as the topic of “covering ears”: “As for me, (I) cover my ears.”

Could we use うるさい instead of 大きい? What nuance would change?

Yes.

大きい neutrally describes volume (“loud”).
うるさい means “noisy” or “annoyingly loud,” adding a sense of irritation.

How might this sentence be expressed in casual speech?

You could switch to から, drop formal endings, and even omit some particles:
• ピアノの音が大きいから耳をふさぐ。
• More colloquially: ピアノの音うるさいから耳ふさいじゃう。