I had that problem until I looked at https://github.com/googlecreativelab/teachablemachine-community/blob/master/snippets/markdown/audio/tensorflowjs/javascript.md 3
which passes undefined in the following:
const recognizer = speechCommands.create(
            "BROWSER_FFT", // fourier transform type, not useful to change
            undefined, // speech commands vocabulary feature, not useful for your models
            checkpointURL,
            metadataURL);

https://github.com/tensorflow/tfjs-models/blob/master/speech-commands/README.md 1 states the default is to add 18 words.