How do you put music in the game?

how can I put different music in the game? it is that I want to put the Stardust Speedway in Metal Sonic Battle

Are you asking putting new music into the current game or music for a metal sonic battle wad

You can open the music.dta file with any WAD editor such as SLADE (reguardless of the file's extension). Then you can go around the entries and replace the musics that interest you even with mp3 files or pretty much anything by right clicking on the entry and selecting Import. To find the music that you want to replace, you can use SLADE's music player to listen to it. The Metal Sonic race should be O_MAP25M, though.
(Note that D_ musics are midis and O_ are the normal ones, so you shouldn't really have to touch the former.)

Again, although you can import mp3 files, the wiki has a full tutorial on how to make proper music for the game, which includes looping.

And the last thing is that you can add music / sound only WADs even in netgames, they don't count as ressource modification.

Hope I've been helpful.