When you upload your .mp3, in it’s present (broken) state, the site will automatically imbed code that looks something like this in your “create topic” screen:
The result will be a downloadable hyperlink like this:
Celebrity Chef_Mix 1.mp3 (11.3 MB)
The only part of the above code that you want to keep is this:
Obviously, the specific file identity will change, but this is the format required.
Now this is a vital step:
Discard the rest (all the code outside the red rectangle) and then put http://indierecordingdepot.com/ in front of what is left.
The result will be a line of code that looks like this:
…Now the site player should be embedded with your song like this:
Hope that helps.