How does speed listening work? I've recently gotten into listening to podcasts. Over time, as I get accustomed to the speaker's voice, I'm able to increase the speed of the podcast to as high as 3x speed. It still feels "normal" to me, as if the speaker is having a normal-speed conversation with me. 

When I turn down the speed to 1x, it sounds unbearably slow. For non-speed-listeners, listen to something at 0.5x or 0.25x, and you'll know what 1x sounds like to speed listeners. It's as if speed listening shifts up a listener's range of acceptable speed, so that lower speeds are no longer in that range.

I know there are plenty of other speed listeners who do the same thing (just Google "speed listening" to see for yourself). Yet, others have told me that at this speed, podcasts are basically unintelligible.

Thus, I have a handful of related questions:


What occurs in the brain when a person speed listens? 

a. Why does the podcast feel "normal" at such fast speeds? 

b. Is there a fundamental/biological difference between those who can speed listen and those who can't, or is it a learned ability?
Is speed listening detrimental to listening comprehension?
Your question made me think of JAWS, a screen reader for the blind. I have worked with visually impaired people for a while and I have always wondered how on earth they can understand the speech produced by JAWS given the sheer high speech rates they apply on their gadgets.

Indeed, people with peripheral vision loss may learn to understand spoken language at rates of up to about 22 syllables per seconds, while the rate in normal listeners is about 8 syllables per sec (Dietrich et al., 2013). These numbers are based on speech understanding, as opposed to simply hearing it.  

Hence in answer to your question


  Is speed listening detrimental to listening comprehension?  


I would say no, at least not in above study, as speech understanding there was tested in a speech comprehension test, which is different than just hearing the speech.

Then your questions 


  What occurs in the brain when a person speed listens?
  a. Why does the
  podcast feel "normal" at such fast speeds?
  b. Is there a
  fundamental/biological difference between those who can speed listen
  and those who can't, or is it a learned ability?


In case of blind people it has been shown that they activate their visual cortex when speed reading. This, because the deafferented area is recruited for other modalities, such as touch and hearing. This is probably not what you are after in your question though. In effect, I doubt gross effects on brain activity occurs in trained, normally sighted subjects and indeed a study where both sighted and blind individuals were trained on fast-speech understanding does not report on gross effects in the sighted group (Hertrich et al., 2006). 

It feels normal because it is a learned ability; to continue with the example of these blind folks - they haven't been able to read for a long time and the only way they communicate through written materials is via screen readers. The spoken word is slower than reading so they crank up the speeds whenever possible, so they are slowly increasing their speech understanding skills. 

References
- Dietrich et al., Front Hum Neurosci (2013); 7: 701
- Hertrich et al. Brain and Language (2006); 124(1): 9-21