Top-left, below the transport controls there is a vari-speed control slider
right-click and change the mode to to “wheel” That way any speed selection will remain (since Ardour 6.6. also as default after play/stop/play).
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PS. For podcasts you may want to additionally consider adding x42 Vari Speed Anti Pitch to Ardour’s master-bus. That plugin counter-balances pitch changes when using Ardour’s vari-speed wheel.
Oh nice! This would probably be perfect although unfortunately on this 6.6 install, it exhibits an extremely weird bug where playback starts at normal speed (or thereabouts) but then actually reverses direction back to the start point, and when it gets close to that, reverses direction again, but with playback slower this time. The net effect is that the playhead looks a bit like a bouncing ball would if gravity was pulling to the left instead of down, with ground level being the start point.
Yeah coincidentally I found that about 30 minutes ago but had to disable it because it was causing strange displacements of the metronome.