How do I make Audition playback really slowly in Waveform mode?

Go to Edit>Interpret Sample Rate. Select a number much lower than your current sample rate, and it will play at a rate determined by the formula New Sample Rate/Original sample rate. So if the original is a 44.1k file and you set it to play at 11.025k then it will play at 11025/44100 = 1/4 the original rate. Yes it will go lower than that, but you are very unlikely to determine a complex rhythm pattern speed if you do. The rates listed are what your sound device will cope with - entering other numbers will probably result in no playback.