Hymns can be found on the Bible Study page.
I Hear America Singing. I cannot emphasize how much we love this book and CD. The CD sat in our car’s player for about 4 years without being rotated! Classic American folk songs appropriate for kids, but performed at a high level and enjoyable for adults.
If you really want to dig into American folk, the best resource is Smithsonian Folkways.
The past few years I’ve simply created a playlist on my Amazon Music account with works from a certain composer, based loosely on Ambleside Online’s Composer Rotation Schedule.
By Period, on Naxos
Free online sheet music grouped by composer: MusOpen