If I'm listening to Andy Weir, it's enjoyable and completely manageable at 2.5x.

If I'm listening to anything involving math or physics, 1.5x is typically a ceiling for true comprehension.

Honestly, it all depends on a few important factors: first, a lot of audiobooks are read at a speed that genuinely feels absurdly slow, as though they are chugging cough syrup before recording.

Next, don't discount what I said about high-quality earbuds; anyone that tries listening above 1x on a speaker is doing it wrong whereas having the sound in your ear makes >1x listening easy. You won't get it until you try it, and then you'll feel the same smug superiority I do, I promise. It's a night-and-day change.

As for the rest, including the listening-while-cycling thing, it all comes down to practice and comfort. Like all things worth doing well, listening effectively in the way I describe is a skill. Your reaction to me listening to a book while cycling seems like how I feel about people driving while talking on the phone; I think it's insane and horrifying, but people roll their eyes when you imply that it seems like it's surely dangerous.