Hi Ken, no there's no cost involved, and yes, you'll be fine with Boxnet.
All you need to do is upload your mp3 to Boxnet, best to create a separate music uploads folder, and then copy the link from the info Boxnet gives you about the file, and then paste the link in your post.
There's a more thorough explanation here:
How to create a free, file-sharing account
If you have any problems, just drop a message back in this thread, and I'll try and help.
Cheers
Paul