When I was working nights, and it was summer, so I wanted my windows open for the breeze, I had a CD of budgies chirping that I would put on continuous play, so I could sleep. It covered any outside noises (white noise). Budgies sound like sparrows, which I find soothing. I think they're a happy sound. Plus down in my lizard brain they probably mean I'm safe, as they would shut up if any predators came by.
--I tried looking for a rainfall CD first, as I find that soothing also, but every CD I tried (I bought 2) at some point started to play thunder which would wake me up. -You would think white noise CD's wouldn't have thunder.
--I also tried music but I would have dreams involving the songs and after a while I didn't find it very restful. -Tori Amos gave me some really strange dreams.