You would upload it just like a picture. However, you cannot upload mp3 files without some trickery. Assuming you have Windows XP or newer, perform the following:
1. Right-click on the mp3 file and move your cursor over "Sent To".
2. A submenu should appear. Click once on "Compressed (zipped) Folder". Follow any instructions (None may appear, but some may) that appear.
3. A new file should appear in the same folder as the mp3 file. It will have the exact same name as the mp3 file but will appear to be a folder with a zipper on it. You can upload that, a compressed folder with extension .zip
To post, do the following:
1. Start a new thread or new reply as normal.
2. Below the text input box look for a button labeled "Manage Attachments". Click it.
3. A window will pop up. In it are a few things. Look for a button (There are several) labeled "Browse" and click it.
4. Navigate to the folder where the mp3 file and the new compressed folder are. Double-click on the compressed folder when you find it.
5. Text should appear in the box next to browse. This is good.
6. Click the "Upload" button next to the lowest "Browse" button and allow the upload to progress. If the file is larger than 5.00 MB it will fail.