Hello!
I’m using OSX Sierra and Audacity 2.1.2m and installed using the .dmg installer. I have a voiceover track with several short speaking parts. I want to add music to these. I recorded the voiceover track in mono. I saved it as a project, then deleted it from Audacity. Then I tried to get it back, but I’m getting an error message that it’s open in another window. I closed Audacity and double clicked on the track but it still didn’t open, only the Audacity shell appeared, and it is not open in another window. How can I open the track, how can I add a second track with music, and how can I blend the two into one track? 
Make sure you have 2.1.2 from http://www.audacityteam.org/download/mac - 2.1.2m isn’t our software.
If you deleted the track using the [X] button, the project is still open. Use Edit > Undo to restore the track. You can also COMMAND + W repeatedly to close all the Audacity windows, to be sure none are open. If Audacity asks if you want to save changes, say Yes. Then use File > New or File > Open in the menu bar at the top. See File Menu for more explanation.
To add music, import a song file from your computer - see Importing Audio. The song will automatically mix with your spoken parts. You can use Time Shift Tool to drag the spoken pieces along the Timeline.
You may want to read Mixing a narration with background music for more tips.
Gale