MIDI/MP3 to AT3 Music converter?

You can't convert from MIDI straight through to AT3, because MIDI is a sequenced format, which needs to be recorded to .WAV before being encoded to MP3, AT3, etc.

An excellent piece of software for MIDI recording/mastering is Synthfont. It makes use of soundfonts to enable you to record your MIDI compilations with whatever soundset sounds best. I'm assuming that right now, like most people, you're using the standard, built-in MS Synth for MIDI playback, right? It gets m.u.c.h. b.e.t.t.e.r.

As for converting .WAV to ATRAC3, you can use the official ATRAC3 codec. Unzip and right-click the .INF file and click install; That's all there is to that. This is the only real way to do it without Sony's NetMD/SonicStage software... Here's a bit of a rundown on the procedure:

After installing the codec, and after recording to .WAV format, open up the .WAV file in something that can save the file - Sound Recorder works in a pinch:

http://runefox.net/atrac3/1.png

Open up the .WAV file in Sound Recorder; In this case, Opening.wav:

http://runefox.net/atrac3/2.png

Go to File, then "Save As..." - In the new dialogue, click the "Change" button, and from there, select the ATRAC3 codec, along with the options you wish to encode at.

http://runefox.net/atrac3/3.png

Rename the extension to .at3 and change the type from "Sounds (.wav)" to "All Files (*.*)". Click Save; It may take a while, but don't worry; It WILL save.

http://runefox.net/atrac3/4.png

Open up the new .at3 file in Windows Media Player (or any other player) - Hooray!

http://runefox.net/atrac3/5.png

Hope I've helped! Also, if you want, I can help you record your MIDI's. I've got a good bit of experience with it and a customized soundfont that, in my opinion, sounds pretty good.