Hi Dale,
Theer are lots of ways you can do what you want.

To understand the process think of the old pianola days. There was the paper pianola roll istelf, and the actual pianola which obeyed the instructions on the roll. The sound came from the pianola, not the roll, the paper roll was simply instructions to the piano to play certain notes.

And so it is with Midi and audio sound.

The midi file is like the pianola roll. A series of instructions to tell some system what note to play. You need some instrument to receive those instructions and actually make the sound.

There are several options here:

(a) The cheapest, and no cost way is to play them through your sound card (the sound card does a midi to wave convervision like the pianola). Then capture the sound that comes out of your sound card. There is a very good program to do that (and its free) called Audacity (see www.audacity.sourceforge.net). Audacity can capture the sound output going through your sound card (You may have to experiment with the Audacity 'line in').

(Incidentally Audacity is a great program to have, as it also allows you to edit your wave file (eg trim of unwanted starts and ends, normalise the sound etc) and also converts from WAV to MP3. And the price is right - FREE!!)

(b) You can play it directly into a keyboard, and then record that sound somehow, and then convert it to MP3 using Audacity. You might record it on a cassette tape, and then replaying it back into your computer using Audacity to capture the sound coming in. There will be a bit of loss of quality going from digital to analogue and and then back to digital, plus the hiss introduced by the tape. But I hope you can get the idea.

(c) Use a Software soundcard.

The cheapest of these I know is the Roland VSC (see www.pgmusic.com)

If you are into orchestral, or big bands or jazz bands, then the Garritan software is great, and it all works with Composer (see http://www.garritan.com/ - it is pricey, and requires at least 1GB of RAM)

If you are into organ music (as I am) I can recommend the Hauptwerk software from Crumhorn-labs (see http://www.cyberhymnal.org/) . Again it is not cheap and requires at least 1GB of RAM.

(NOTE: like with the pianola system, the quality of the result depends on the quality of the piano, not the pianoloa roll. So with midi/MP3, it is the quality of midi to WAV convertor that determines the quality of the result).

A search of the internet using Google (Midi to WAV) will reveal a lot of software out there, and then use Audacity to convert WAV to MP3.

I hope that helps you get started - but have a look first at the free option just using Audacity (you need to also download a free MP3 encoder for the conversion to MP3 - but it is explained in the Audacity system).

Cheers ... Clyde