Install a digital audio workstation (DAW) on your computer. These programs allow you to manipulate music files using an array of editing options. If you cannot afford a professional audio editor like Logic or Cubase, download a free program like Audacity, Traverso or MP3DirectCut (see Resources).
Open the MP3 using your digital audio workstation. Open the software and wait for the editing browser to appear. Drag your MP3 file directly onto the browser and drop it anywhere in the blank window. It will appear as a long horizontal bar with wavy lines representing sound waves.
Change the pitch of the MP3 file. Raising the pitch will change the song to a higher key; lowering the pitch will change it to a lower key. In the digital audio workstation, look for a button or meter that reads "Pitch" or "Key." In some cases, it will be in the "Options" panel on the menu bar. Move the slider to change the key, or choose a key like "C" or "F#" from the chromatic scale. Raise the pitch liberally to create a chipmunk effect, or lower it by an octave or more to create a spooky, demonic sound.
Apply effects to your MP3. Locate the "Effects" or "Filters" option in your DAW and select the desired options from the list. You may be able to access the Effects panel by double-clicking the MP3 in the main window. Depending on the software, you will be able to add reverb, distortion or a phaser effect. You can also boost the amount of bass or eliminate the vocals from a song. Try different effects to see what works best. If you decide that a particular effect does not work, just deselect it.
Delete sections from the MP3. Use the "Split" or "Cut" feature to get rid of an unwanted portion, like an extremely long bridge in the middle of a song. Locate your playhead, the vertical line marker that moves across the browser during playback, and drag it with your mouse to the spot where you want the deletion to begin. Click the "Split" button in the DAW (your DAW may call it "Cut," or use a small pair of scissors as an icon) to break up the track. Do the same at the end of the unwanted section. Click the file between the two "Split" lines and remove the region by clicking "Delete." Use your mouse to drag the beginning of the second region to the end of the first region, filling in the gap.
Save the edited MP3. Click "File > Save" and choose "MP3" as the format.