How to mix multiple midi tracks to a single audio track?

Can  someone help me understand the best way to mix multiple midi tracks down to a single audio track? I have tried this by creating a new audio track, arming it to record, then I tried to  select multiple tracks as the input, but it keeps defaulting back to the stereo microphone.
A related question: I have midi tracks playing through separate soft synth tracks. Should the input for recording to audio be the midi track or the soft  synth track?
Not sure if I am going about this the right way.
Thanks, John

Thanks Kalle. I just tried file/export audio and that seemed to work.
I do try to read the help files, but sometimes they use terms I don't understand.

I'm not sure I understand all the differences between exporting to audio, freezing and bouncing. Exporting to audio worked for what I was trying to do, which was to create a single file that I could put on my ipod and share with people. What is the typical reason you would want to bounce or freeze tracks?

Freezing tracks temporarily bounces the synth tracks to audio, releasing the synths and freeing up processing power.
Bouncing the tracks to audio allows you to permanently render synth tracks to audio, maybe to free up synths for more tracks, or free up tracks if you're approaching the track limit. If you're composing a symphony and have 16 string tracks, 14 horn tracks, 10 woodwind tracks, you can bounce each instrument section to a single track, i.e. 1 strings track, 1 horns track, 1 woodwinds track, allowing you to use those tracks again for other instruments.
If I had a situation like that, I would bounce the instruments to single tracks, then export those instrument tracks somewhere else for safekeeping, delete the single tracks from the project and re-use those tracks for more instruments.
Having everything rendered to audio allows you to use audio effects and you can see in the waveform whether anything is clipping.
Bouncing an entire project to audio lets you see if the track is clipping and lets you hear how it all sounds as one song (muting the other tracks).
It will also allow you to export the entire project as mp3, if that's what you want to do, by selecting just that one bounced track.
Audio files are large, there's no way of getting around that, but with the huge hard drives available on today's computers, file size will only be a factor if you're a professional sound engineer doing a lot of work or are an extremely prolific song writer.