Here's an Arminian site with a few free mp3 files: http://www.imarc.cc/audio/audiodex.html
There is an excellent audio critique of Calvinism by Dr. Jerry Walls (co-author of Why I'm not a Calvinist). There are several sermons by Dr. Vic Reasoner, who helps produce The Arminian Magazine. In addition, there are 4 John Wesley sermons that have been narrated by Rev. D. Crossman.