I pretty much always have my ipod on full volume in both ears, which isn't as loud as you would think (I might need new headphones), as I can still hear everything I would normally hear around me e.g. traffic noise and pedestrians talking.
I cycle to work on busy roads with cars everywhere, so visual checks seem to be the best way of staying aware of whats going on, for me, whether I am listening to music or not.
I have always tried to keep constant vigilance of what is happening on the roads and footpaths around me, using my eyes and sense of smell (lol), unlike many drivers, pedestrians and other cyclists I encounter, who make turns and crossing without even a glance at what is going on nearby.
If I am cycling out of the city, I don't listen to any music, as sweeping roads and hills can hide all sorts of speedy dangers,
I would suggest not listening to spoken word comedy whilst driving/ cycling, as I almost crashed my car whilst listening to
On The Hour a couple of days back, as I was having laughter fits and wasn't paying any attention at all the a large cement mixer lorry that was pulling across my lane...