Further research has led me to several possible options:
- Simply using the Gain slider. I think this is meant more for making one track louder or softer than the other, but I think selecting both tracks before using it would enable me to increase or decrease the volume of the whole file.
- Using Effect > Amplify and inputting a negative number.
- Using Effect > Equalization perhaps.
- And finally, using a different program altogether. This 2013 forum post https://forum.audacityteam.org/t/how-to-decrease-volume-of-a-music-file/28979/1 mentions a program called MP3Gain, which might make it easier to make the volume of all tracks in a playlist similar. MP3Gain does have a Mac version http://projects.sappharad.com/mp3gain/. It would be wonderful just to do this at the push of a button without importing or exporting, although I do worry a bit about downloading programs I’m not familiar with.