Learn Italian: Siamo tornati a sentire un affiatamento indissolubile.

The structure tornare a followed by an infinitive is a common way to express that an action is happening again. While you could just use an adverb like di nuovo (abbiamo sentito di nuovo), using tornare a literally means "to return to doing" something. It gives this sentence a slightly more poetic or emotional nuance, emphasizing the return to a previous, familiar state of harmony.

Since sentire takes avere, why is the auxiliary verb here siamo instead of abbiamo?

With this specific phrase, the auxiliary (helper) verb is determined by tornare, not by the infinitive that follows it. Because tornare is a verb of motion that always takes essere, you must say siamo tornati a sentire, even though sentire by itself would take avere (abbiamo sentito).

Why does tornati end in -i if it's followed by an infinitive?

Whenever you use essere as your auxiliary verb in a compound tense (like siamo here), the past participle must agree in gender and number with the subject. The -i ending on tornati tells us that "we" refers to either a masculine or mixed-gender plural subject. If a group of women were speaking, they would say siamo tornate a sentire.