Why would you want to convert a MIDI file to the WAV format. There's at least one advantage: WAV files play more identically across systems and sound cards than MIDI files.
If you are not sure about the fidelity of the sound card in a delivery machine, you could always convert your MIDI into a WAV - although in the process you will lose all the inherent advantages of the MIDI format, such as small file size and lower consumption of processor resources. Another often overlooked advantage of using a MIDI file as a background score is that you can use WAV files simultaneously for other purposes such as narration or event sounds - almost all sound cards can play one instance each of a MIDI and a WAV stream simultaneously.
If you still feel the need to convert your MIDIs to WAVs, there are at least 4 of these 'Midi To Wave Realizer' programs which do the job - one freeware and three shareware.
This is an online MIDI to WAV converter that does not require any installation on your computer. There is a limit to the amount of content you can convert each day, but if you need to convert a file or two, you will most probably not run into any such limitation.
Go to the Online MIDI to MP3/WAV Converter site.
WinGroove is among the most well known of these programs - with this program the MIDI output of your ordinary sound card will sound almost as good as that of a high end card - but beware, WinGroove eats into your processor resources like someone who has been starving for a week. The reason is that behind the scenes, WinGroove converts your MIDI stream into a WAV stream realtime. So it may entail better sense to actually open a MIDI file in WinGroove and convert it into a WAV file - after that your WAV file is like any other WAV file.
MIDInight Express from Polyhedric Software is a small download. It functions as a player for MIDI and WAV files. More importantly, it renders or realizes your MIDI stream into a WAV stream - it is also the least resource hungry of all applications discussed here. What's more - it's freeware too - but read the help file thoroughly before you attempt to convert a MIDI file to WAV using MIDInight Express - this is not the easiest of applications to use. You may require PRG files (the help files explains everything) to effect a conversion.
Midi2Wav Recorder is a shareware application - a demo download is also available. It offers loads of features not available in other applications.
Timidity95 is a free program originated in Japan - the original developer stopped development of the program. Since it is a GNU program, an alternative developer has created a new version. Timidity is not for the faint hearted - there's no help file and the entire documentation is available online at the site. Apparently, the program is quite popular in Japan, since the site maintains a Japanese newsgroup on Timidity.
The Ultimate MIDI page is a great place to learn more about MIDI stuff.
Go to Computer Music UK to learn more information on MIDI and other sound matters.