Learn Swedish: Det låter bra!

In Swedish, det is used when referring to a general idea, a suggestion, or an overall situation rather than a specific object. Since you are reacting to a plan or a concept rather than a specific noun, you use det.

Can the verb låter be used for actual physical noises too, or only for ideas?

It is used for both, exactly like the word 'sounds' in English! You can say this sentence when you like a plan, but you can also use låter to talk about a physical noise, like a bird or a musical instrument.

Since the sentence starts with det, why isn't bra spelled with a T at the end?

Usually, adjectives get a -t ending when they match a det subject. However, bra is a special exception in Swedish. It never changes its form, so it stays exactly the same for en words, ett words, and plurals.