I personally like Shakuhachi a lot. It’s a particular style of wooden flute. Give it a listen sometime. I’m not sure you would call it relaxing, but it can be. Sometimes it’s kind of intense.
As far as meditation, Japanese monks don’t listen to music when they meditate…though they sometimes have a time drum which has very slow roll downs to let them know how much longer they have to meditate. They do this at Green Gulch Farm Zen Center, which was founded by the late Shunryu Suzuki, a Japanese zen master.
If you’re looking for something really peaceful, try meditating at the ocean with no people around. The incessant crashing of the waves is a great relaxing sound. Personally, I also like meditating in forests with a lot of birds and some light breeze. The natural sounds are beautiful but they keep me aware so I don’t daydream too much. Alternatively, meditating during a thunderstorm can be amazing.
Cheers.