Firstly, you'll want to download it and host it on a site that can be embedded on the forums using the media tag (or have Banksy_Pie host it on one of those sites). Sites that come to mind immediately are YouTube, SoundCloud and Pony.fm; all of which can be embedded here on MLP Forums.
And yes, you read that right. Pony.fm songs can be embedded here on the forums on any post simply by copying and pasting the URL for the song's page. Example:
Since that song's for an OC, it qualifies for Pony.fm, so I think your best bet would be to have Banksy_Pie sign up for Poniverse if he hasn't and upload that track so you can use it.
Quality control is an ongoing discussion. You can read the lengthy Featured Artists thread here and my proposal for a unified spotlighting/feedback system here.
DusK's post in Mail Mare Repeated MSG and Embebed Audio Posibilities was marked as the answer