Harvest Moon: Animal Parade Discussion

I was just wondering if the music changes with the seasons in this game?

Yes it does.
It also changes for certain festivals and every season has a second music track that plays at night!

Are there any easy ways to quickly increase relationship levels in the game?

There are if you give residents the proper gifts, but that's mostly guesswork.
Every heart above the resident's head when you give them a gift is 1% of the towards earning another heart in the relationship status.
Bad gifts can decrease your relationship by up to 10% of a heart.
The best gifts can increase your relationship by up to 30% of a heart. =)

Has anyone managed to get all of the young adults married in your town?

That's not possible unless you don't marry anyone.