Thanks to this thread, I can attest that the problem with hearing no audio while using Varispeed Speed Only mode is the Input Monitoring option being selected!
It's not really discussed as anywhere that Input Monitoring will cause a problem with Varispeed, but I guess the manual at least gives the following warning (from the
Work with Tracks section https://support.apple.com/guide/logicpro/turn-on-input-monitoring-for-audio-tracks-lgcpbfbefa96/mac):
Since others might be similarly frustrated when these interactions are not documented, I thought I would also add the error message I get when toggling input monitoring on and off (which message I find somewhat puzzling, but I'm guessing will make someone here go, Ah!)