I understand the whole context and the words meaning, but am confused with why "they must have listened", and particularly what does "something" refer to in the sentence? My guess is "some professional, different types of music"? But then it leads back to why "they must have"?? (my guess of "they" probably referring to the orchestra players they have just listened to )
Here is the context:
We sat in silence, gazing at the castle. Normally, at night, it was bathed in a kind of orange glow from the lights dotted around the fortress wall. But tonight, under a full moon, it seemed flooded in an ethereal blue.
‘What kind of music would they have played there, do you think?’ I said. ‘They must have listened to something.’
‘The castle? Medieval stuff. Lutes, strings. Not my cup of tea, but I’ve got some I can lend you, if you like. You should walk around the castle with it on earphones, if you really wanted the full experience.’
The context is : The two people sitting in their car chatting, just coming back from a concert which they both enjoyed, still lost in the remembered music.