Load the audio file you want to make smaller by using an audio editing program. Windows does not have an audio editing program by default, but software applications like Audacity are free to download -- commercial programs like Adobe Audition and Sony Sound Forge will have more options for reducing the file's size.
Make the audio file smaller by first trying to reduce its length. This is a great idea if you need to send only a certain section of the full track. Do this by clicking and dragging the cursor over any sections of the audio track you don't need, for example any unnecessary intros or outros. Remove the unwanted parts by clicking the "Delete" key on your keyboard -- move on to the next section if there is no audio to trim or remove.
Click on the "File" and "Save As" options from the command menu. The program should open a new window with a file explorer display for saving the track, but also another series of options for its quality.
Look at the options and find a dropdown menu for the bitrate. Bitrate is measured in kbps and determines the sound quality of an audio file. Click on the dropdown menu and select a bitrate that will save space but not compromise the audio quality too much.
Choosing a bitrate like 160kbps will compress the audio by a ratio of roughly 11:1. High-quality sounding MP3s are compressed at 320kbps, which will compress the data at a ratio of approximately 4.4:1. Although an audio track saved at 160kbps will compress the data more and save more space, it will affect the audio quality, namely in the detection of certain instruments and slighter sounds.
Check the "Mono" box if you want to save even more file space. Mono simply means the track will play as a linear audio file exactly the same through multiple speakers. The opposite is stereo, which takes up more space because it divides the instruments and sounds between speakers, producing a dynamic surround-sound effect.
Save the file and name it something new to compare sizes with the original track. Doing so also means you can go back and edit the original if you are unhappy with any reduced audio quality.