Can I use my passport for ID to get into the clubs there? Just realized my DL is expired and may not have time to get it renewed before we get there.

It is a valid ID. No reason you can't.

Just Depends on days sometimes i.d sometimes not

We were carded at every door and we are both over 40.

I was in Nashville at the end of August, I am almost 40, our group which I was the youngest of was carded at almost every establishment we went to, probably 90%.

Probably. If you look older than 18 you probably won't get carded anyway. My 15 yr old brother walked right in all the bars to listen to the bands. And me and my husband stayed for several days and never got carded once.