You can easily record and insert an audio file of your voice on a PDF.  This can be used to provide instructions for an activity or provide students with feedback on assignments.  The next time you open a PDF, any PDF, try the following. . .  

Go to Tools > Comment & Markup > Record Audio Comment  and a little recording icon will appear > Click somewhere on the page to place the file > Click the red dot to record >  Start talking, right at your laptop (there is a mic by the web cam) > Click OK when finished recording > OK

 
Set Audacity to export directly to MP3:
If your students use Audacity to create audio files, there is a way to set it up so they can export directly to MP3 as opposed to using Format Factory to convert.  View instructions here.