Quiet music makes no sense. Soft music, yes. Quiet music, no.
adjective, quieter, quietest. 1. making no noise or sound, especially no disturbing sound: quiet neighbors.
Silence speaks louder than words... but we don't really say "silenter" and "silentest", do we? How many times have we asked someone or been asked to "be quieter"? It's not a request for absolute silence, just to turn the noise down a bit.
I'll add though that while Google finds ~350,000 results for "quiet music", it finds more than 3 million for "soft music". I would definitely argue for making "soft music" the primary translation.
What I was wondering if it was intentionally translated that way or if it's a mistake of some kind. It just sounded a little too abstract to me although it's not uncommon for Duolingo to put together sentences that make no real sense.
That makes me wonder: do the Russians literally say 'quiet music' and is that really a thing in Russian?
As a musician, I know this incredibly dense to say, but quiet music is not necessarily soft. Quiet is loudness whereas soft refers to intensity. You can have intense quiet music, and soft loud music (blasting Michael Buble at the neighbor's for example). I'm assuming there is a distinction in Russian too, but I could be wrong.
I suggest listening to native-speakers pronounce тихая. It sounds a bit different from the computer voice:
I cant be the only one who accidentally hits the "cant listen right now" option. DUOLINGO. PLEASE allow users to toggle or undo this option withouthaving to leave the lesson. Preferably an "undo" button on the pop up page that says listening activities will "return in an hour".