Given your midi files are linked to vst instrument(s) with the correct midi channel(s) assigned to them, to export your midi to audio you go to the file menu on the top left in Acid and selecting Render as, which will bring up a window where you can make bitrate and file format selections,name the file etc. Hit ok or press enter when done with your selections and you 're done. If you want to export a single midi track from many you have to solo that track then go to Render as, like before. Hope this was helpful to you, cheers and happy new year :)