I want to export some of my works to a MIDI file, but whenever I hear the final copy, all of the instruments reset to a piano. How do I fix this, I have the free version of Mu.Lab.

MIDI doesn't contain any sound.

There is a MIDI message, "Program Change", that can be sent to a MIDI device to tell it to select a particular "program" or configuration. If the MIDI device is a General MIDI Level 1 device for playing notes as sound, then the list of Program Change messages matches the GM1 sound set. Not all MIDI devices use Program Change messages, not all that do use them map them to the GM1 instrument set.

So... How do you insert a "Program Change" in MULAB? Open the sequence, go into the List Editor, click the "Insert" button and choose "Program Change".

I cannot seem to find what you are specifying, where can I find it.

Here you go:
Insert the Program Change in the List Editor

and set the Program Number


You can set the Time when the Program Chnage will occur by clicking the "Time" in the List Editor.
Please note that the program change in Mulab starts at "0". Often the program number is interpreted as starting from "1".
So, Program 24 will give Program 25 (in the General Midi setup that would be an Acoustic Guitar)

OK, but one thing, do I have to know which program number I want to change it to, or is it already chosen.

You say that with your midi files all tracks play the piano sound. This tells me that you have other sounds in mind. Which program numbers these sounds would have depends on the device playing your midi file.
If i assume General Midi, there are fixed sound set lists as standard. Google general midi sound set, or just step through the sounds of your playback device, each step is +1 of the program change number.

I linked to the list of GM1 sounds above.

I you use a GM vsti in the rack than it might been possible that the loaded midifile in MUlab has the right soundassigning?

But there are not so many GM vsti to find

Thanks, guys, for the help. This will be handy.

However, there seem to be more instruments in the Mu.lab collection of instruments than MIDI devices, would I have to export it as a different kind of sound file for the instruments to work properly. If so, how.

If you want to output to a file the audio you hear from MULAB, you want "Session->MixDown Audio".

Alright, then, I was confused on what that meant. Thank you, that is all I need to know.