This question has the good recommendation of Audioxplorer for real-time spectrum analysis that doesn't require you to have a mic in or anything else - it just analyzes whatever sound comes in your standard mic. Unfortunately, this is not only no longer developed, it doesn't work on my new Big Sur/M1 chip.
Any recommendations for free oscilloscope/spectrum analyzers with anything like the capabilities of Audioxplorer? I assume the previous question is old enough now that a new question is probably warranted. (Something that involves slightly more setup, but not too much, is also acceptable; but AX was so great - just open it and you show your students what the waves actually "look" like!)