A favorite topic of mine. Post Peak Music that is, not COC minutia. Most people have a very sketchy idea of what traditional culture was like.
Most singing was a capella, not that they called it that. No strummed guitars. People would sing around the house or out in the fields. Some of these songs have been around for centuries, too.
Ozarks singer
Almeda Riddle singing the Banks of the Yarrow, an old song from Scotland.
BA Botkin's books like
A Treasury of American Folklore have a good cross-section of information, stories, songs, games, what have you. They're available used very cheap, too.
Instrumental music will be up in the air, I think. People will strum guitars or play violins for a while but what do you do when all of your strings break? Know anybody who can make new ones? I've read about gut stringmaking and it's awfully detailed work. Maybe people will make new metal strings - that actually might be simpler for the time being.
Leftover metal instruments may have more of a future - trumpets, trombones. Woodwinds need an awful lot of pads for their keys though, and need occasional maintenance to work properly. There are simpler remidies though like bamboo saxes with only two/three keys, or the old simple flutes/fifes which are often just a tube of bamboo or cane (similiar to bamboo, it's what you make woodwind reeds out of, also) with some holes punched in it, like the cane fifes you hear in Gangs of New York - that's Otha Turner, from Mississipi.