Use Server.MapPath: It converts a relative-to-your-website path like "~/uploads/MP3/" to a full path specification you can use in the File.Exists method:
if (File.Exists(Server.MapPath("~/Uploads/MP3/" + filename)))
{
... do something with the file
}
You would only need an absolute URL if you are planning on putting a permanent link to the file directly onto the clients webpage (which isn't a good idea, as it means the folder must be publically available).