I stumbled upon this question about a week ago, "I wonder, could I possibly add my own music so it plays in-game?" Of course, you can do any music, but I was thinking more of the songs by C418 that AREN'T in the game (I.e dog). So I set out to experiment, and came up with a "mod" I guess you could say. A manual mod O.o
1. Get the music in which you want in-game. I believe it can be in any format you chose, doesn't matter whether it's .mp3 or what.
3. Here at this site, you want to navigate to the "Choose file" button and find the music of your choice. I recommend that you put the music on your desktop for easy access.
4. Once the file is chosen, press "Convert File" This will format the music to .ogg. Once converted, it will automatically download the new file for you (your music in .ogg) You can find it in your downloads folder (or whatever folder you use to download, which I'm assuming you know...)
5. Alright, now you have your .ogg file. Navigate to the start menu, and type %appdata% into it. Press enter. Go to .minecraft, then resources. Go to "newmusic". This is where all in-game music resides, and you can now drag your new song into the folder.
6. I hope this tutorial was clear enough, feel free to ask any questions. I felt the need to make this as soon as I accidentally realized that this works. I was just playing Minecraft moments ago when my custom music (It was playing Moog City by C418) started playing. I was amazed that it worked and felt the need to share this method. Thanks for reading and if you have any questions, I'm all ears... or would it be eyes?
EDIT: Just in case you didn't know what I meant by "In-game music", I'm talking about the music that occasionally plays while playing the game. You won't see immediate results after doing this, as the songs are played at random I believe. So be patient, for soon you'll hear your music in the game . Add more music to increase the chances of hearing yours! Trust me, it's pretty satisfying (Tempted to put some rock music in there XD.)