Instructions
Adding Sound to a Stage
Open Windows Explorer by clicking on "Computer" in your Windows Start menu. Locate the directory that contains your "M.U.G.E.N" installation and open the "sound" folder inside it. Copy and paste the audio file that contains the sound you want to use into that directory.
Open the "stages" directory inside the main folder of your "M.U.G.E.N" installation. Right-click on the "stagename.def" file, where "stagename" is the name of the stage for which you want to edit the sound, and select "Open with." Click on "Notepad" to open that file into Notepad.
Locate the "[Music]" line inside the file. After that line, you should see two lines, containing "bgmusic=" and "bgvolume=".
Type the name of the audio file after "bgmusic=" and "0" after "bgcolume=".
Save the ".def" file by clicking on "File" and selecting "Save."
Adding Sound to a Character
Launch Windows Explorer. Locate the folder that contains the files of your "M.U.G.E.N" installation and open the "sound" directory inside it. Copy and paste or drag the audio file you want to use into that folder.
Go back to the main "M.U.G.E.N" installation folder and open the "data" directory inside it.
Right-click on "select.def," select "Open with" and click on "Notepad."
Locate the "[Characters]" line in the file, and type or copy and paste the following instruction, on the same line as the name of the character you want to associate the sound with:
Save the file by pressing the CTRL and S keys at the same time on your keyboard.
"music=sound/filename.extension"
Replace "filename" with the actual file name of the sound file and "extension" with the correct extension.