I found a nice piece of software to convert my MIDIs to MP3s. It is called Direct MIDI to MP3 by Pistonsoft. (http://www.pistonsoft.com) Other solutions I looked at required a real-time conversion but not Pistonsoft's solution. You can also batch convert your files. I found this software very easy to use and converted files quickly.
Direct MIDI to MP3 comes with a 30 day trial and costs $29.95 to
purchase. However, if you promote the software on your website or blog
(see the Pistonsoft site for details) they will send you a free key to
register your software.