To overlay multiple tracks to create a single, mono Audacity recording.
Let's say you want a number of different speakers talking or singing at the same time, or you want to add some background music or other sound to a voice track.
A) If it's only a small section of your "parent" recording that's going to be affected, it's probably less worrying to copy the affected section of the parent, open a new file in Audacity, and paste your copy into it (leaving the parent file open in the background, with the affected section still highlit). Then you can tinker with your copy of this section as you like, then copy and paste it back into the parent file, replacing the highlit section.
Otherwise, you can play with the parent file itself, but do save it first, before you start mucking about with it!
B) Put the cursor at the front of the sample you've extracted, or (if you're recording directly into your parent file) near the point where you're going to merge your new tracks into the parent. Now click the Record button and record the sound that you're going to merge. Audacity will automatically open a new track for this recording (which you can see by scrolling down). Stop recording in the usual way.
Repeat this process to create more overlap tracks at this location.
C) Next, you may want to get the tracks approximately synchronised. You can move a whole track forward and backward in time by clicking on the double-headed arrow at the top left of the Audacity control panel (the "time shift tool") and dragging the track left or right.
If you need to make fine adjustments, for synchronising the starts of words between tracks for example, you can cut and paste background "silence" within the tracks until you get the proper match between the tracks.
You can check how they sound, alone or in combination with other tracks, by clicking the Mute button at the left end of each track, so it will play or stay silent.
[I think Audacity is probably not the best software to use if you actually want precise synchronisation of tracks, for a chorus of voices, for example. You could do this by first recording your base music track (or perhaps just a track of timed clicks). Then from the menu, Edit/Preferences, on the Audio I/O tab, check the box "Play other tracks while recording new one", then record your overlap track while listening to your "backing". It will almost certainly take a lot of re-recording to get this right, although you can do a certain amount of adjustment of your timing by cutting and pasting silence into the gaps in your overlap track. *** Do remember to un-check "Play other tracks ..." when you're finished.]
D) Now to fuse the files into one. First highlight each track in turn and, on the menu, click the Project/Quick Mix option to choose the current track for merging; the track header in the control box at the left end of the track will change from "Audio Track" to "Mix" when you do this. Now export the file as a WAV file. This will create a single track, combining all your mixed tracks. Now open this wav file with Audacity.
One minor complication is that, if each track is recorded at a reasonable volume, then, when they are added together in this way, you'll get clipping where the combined recording is now too loud. So, before marking tracks for mixing in this way, listen to the sound of the combined tracks and cut down their output volumes with the slider at the left of the toolbar when necessary.
E) That's it ... unless you still need to cut and paste your new section back into the parent file.