Backgrounds are tough, I usually lean toward public domain images for backgrounds when it comes to visual novels haha. But hopefully you can find some good ones! If you need any help with finding music, I can recommend a few places to find free use (with credit) music!
Of course!
Soundimages has a lot of music, all sorted by genre/vibe
Pixabay also has free to use music
Freesound is great for finding little tunes and sound effects, just make sure you double check the usage rights because some of them have different permissions!
ELV Games has a huge variety of bundles for music, though they do cost money
And of course, there's tons of free music packs on itch.io as well!
I also have used music from Darren Curtis and Aaron Krogh ^^
I hope some of these are helpful!