Minecraft wiki entry for Redstone_music, associated text: Be careful with water as it destroys redstone on contact.
If you have a lot of redstone, it might be helpful to color code or label what part does what in case it breaks.
You can use pistons to start/stop the song.
You can use buttons to start/stop instead of levers.
It is possible to make a song with a certain section that keeps repeating until a signal comes through and moves to the next thing. This is achievable by making a closed circuit using redstone, repeaters, and note blocks, and use a hopper timer to time when to open the circuit and continue the song.