This is a process that involves multiple steps.  Below is a brief summary, with details following. 

Voice Record Pro is a free app for iPhone, iPad, and iPod.  Once you have downloaded the app from the App Store, and have installed the app, go ahead and open it on your phone.  You should see this screen:

Hit 'REC'.  When doing so, it won't start recording.  It will bring you to another menu.

Here is the next screen you will see. It will bring you to the 'Preset' tab.  Go ahead and select 'High'

Now you'll need to select the 'Advanced' tab.

This next screen, is where you will select the file type for your audio file.  You'll need to change it from MP4, to MP3.  Just hit the plus sign.

Now you should see this:

When you are ready to record your message, hit the red 'REC' button on the bottom.  It will start recording right away.  When you are finished, hit the stop button:

After you are done recording, listen to it to make sure it meets your need.  Just select 'Play' in the next screen:

If you feel like the volume needs boosting, select the 'Audio Boost' in the middle of the screen.  I would suggest uploading the file into Tune first, to see how it sounds with our music.  

Once you have finished, you will need to send the file somewhere.  I have chosen to email it to myself.  I selected, 'Send By Email', but feel free to choose any option.  

Now, you'll need to move from your phone, to your computer. 

7. On the left navigation panel, scroll down to find 'Messages'.  
8. Click on 'Your Library'
9. Select the 'Upload' button

10. Find the file you saved earlier and made a note of

Now you can create a 'Message Preset' or 'Schedule a Message'