I have a Foxpro
and have done what you are asking with pretty good success.
has a program which allows you to upload in various audio formats.
All you need to do is get your hands on a decent device that will record in one of those audio formats. In my case I just pulled out my Iphone and used the voice record function. You need to make sure that your recording is fairly loud, but no so loud it gets distorted or you will obviously have some sound quality issues. The Iphone works great for this because it actually has a graph on it which shows how loud it is recording. Then it's just a matter of getting it from the phone into the Foxpro. Easy peasy......
I actually recorded some bird sounds off of a website just playing it through my computer speakers and then recording them with my Iphone and put them on the Foxpro. They sounded surprisingly good and the caller will play them plenty loud.