I have a problem. When i host a server, and my friend enters it he doesn`t hear any music until he downloads it from JJ2 site. Can i make anyone who join my server automatically download music with the level?

Eh no, the music is not needed for the level to work, just the level and tileset. The music can be pretty big (I've seen up to 2mb), that would add a lot to the download time. The best thing (and proper for that matter) to do is to mention the music file in the level somewhere, so the player can look for and download it.

- JelZe GoldRabbit =:3

Well technically it is possible to patch your jj2 to download music files along the level and tileset, but that would only give you a number of new problems like additional waiting for download, server not being too happy about you lagging it with unneccessary downloads, you would get error downloading file and you'd be unable to join the server, should it not have the music file, etc. You can never have all music files, but it's best if you download them from J2O and create a collection yourself. Most of the players likely have large number of music files in their jj2 folder and it is a result playing, downloading and trying out levels from J2O for long periods of time.

And please don't ask me for such patch. I can't create it for you because I'm an idiot