zatuon ga sukunai yoru ha gakki no oto ga kikoeyasui.

This is a typical sentence with a relative clause plus topic-comment structure:

• 雑音が少ない modifies 夜 (“nights that have little background noise”).
• 夜は marks that whole phrase as the topic (“As for those nights…”).
• 楽器の音が聞こえやすい is the comment (“instrument sounds are easy to hear”).

In English word order: “On nights when there is little noise, it is easy to hear musical instruments.”

Each が marks the subject of a different clause:

1. 雑音が少ない: が marks 雑音 as the subject of 少ない (“noise is little”).
2. 楽器の音が聞こえやすい: が marks 楽器の音 as the subject of 聞こえやすい (“instrument sound is easy to hear”).

They don’t conflict because they belong to separate clauses.

～やすい attaches to the stem of a verb to mean “easy to do” or “prone to do.”

• Stem of 聞こえる (to be audible) is 聞こえ.
• Add やすい → 聞こえやすい (“easy to hear” or “tends to be heard”).

Both mean a “quiet night,” but nuance differs:

• 静かな夜 focuses on the overall quietness (“tranquil night”).
• 雑音が少ない夜 explicitly highlights the absence of unwanted noises (traffic, chatter, etc.). It’s more objective/technical.

You don’t literally “hear” the instrument itself but the sound it makes.

• 楽器の音 = “the sound of the instrument.”
• 楽器が聞こえる would sound like you’re “hearing the instrument” as a physical object, which is unnatural in Japanese.

Using ～は on a time expression sets it up as the topic or context (“As for nights with little noise…”).

• 夜に would simply mark when something happens (“at night”).
• 夜は contrasts or scopes the comment specifically to those nights.