I want to send long (20,000 words) audio files to someone who will transcribe them to a word document. I have vista. What is the best way to do this...

You will have to buy or download a separate audio program. I don't know of any way in Vista...
Once you have a program, Convert the file to MP3 but play around with the bit rates and stuff. The lower the bit rate, the smaller the file.

Try using YouSendIt.com. You can upload large files to their servers, then send the recipeitn an e-mail link so they can download file.

What audio format are the files in? It would be better if you can convert the, MP3 since that will compress them.