How to restore music back on my iphone 4s?

Earlier today I backed up my brothers iphone and I don't have any of his passwords because he passes away a few months back. Then went to sync music and it said something like "sync and erase," so I clicked it and it put on all the music I wanted to add and erased all the other songs. How do I get them back, I'm panicking!

You really can't get it back…

Why do you think you can get them back given that iTunes said "synce and erase". Unless the songs are saved somewhere else, they're lost. I'm sorry about your loss, but iTunes said "erase" and you chose that. That's what "sync" means - to get one device to match another. Maybe they were on his computer or saved to CD or something. That's what I do and I've still got music I downloaded over a decade ago.