How can I compress an audio file to under 16MB on Windows?

Windows Voice Recorder to compress audio files isn't really straightforward because it was never designed for that. It's a recording tool, not a compression tool.

So how could you use it to "compress audio files"? You'd have to play your original audio out loud (through your speakers):

Step 1: Press Win + R, type soundrecorder, press Enter (this opens the old Windows XP sound recorder)

Step 2: Record your audio or open an existing WAV file

Step 3: Go to File > Properties

Step 4: Click Convert Now

Step 5: Choose an audio format with lower quality 

Step 6: Click Save As to save the compressed version

Then you save that new recording, and voila—you've technically created a smaller file. But it's incredibly clunky, you lose quality from the analog re-recording step, and it's not a real compression workflow by any stretch of the imagination.