Import your preferred MIDI file into Audacity using the 'Import MIDI' command in the 'Project' menu. After making your selection, Audacity will load the MIDI file and create a visual representation within the main interface.
Separate your instrumental into individual MIDI tracks and import them into Audacity to obtain a greater level of expressive and technical control over your MIDI production. By doing this, you will be able to change volume and equalization settings of particular elements in the overall mix.
Adjust the equalization, or sonic texture, of the MIDI tracks in your production by clicking and dragging over the MIDI data you wish to edit and selecting the 'Equalization...' from the 'Effect' menu. Use the sound spectrum diagram to raise or diminish specific elements of the overall signal. If you wish to lower the bass levels of your production, for example, click on the purple line at the 100 Hz guide marker and drag it downward.
Adjust the volume of your MIDI tracks using the 'Amplify' tool within the 'Effect' menu. Use the horizontal slider in the menu that appears to raise or lower the volume of your tracks. The information displays within the menu illustrate the current level of dB gain or reduction as well as the new peak amplitude.
Add fade effects to the beginning and ending of your track using the 'Fade In' and 'Fade Out' tools within the 'Effects' menu. Select the desired length of time you wish to use as fade material by clicking and dragging over areas of your track. After activating the fade, Audacity will automatically create the effect, manipulating the volume so that it proportionally reduces or augments relative to the volume level you have set for the remainder of the track.
Save your MIDI instrumental in a popular file format such as .WAV or .MP3 using the 'Export As' command within the 'File' menu. This can be especially useful if you wish to share your track over the Internet or through physical media devices.