Why do that?
Slowing audio down can help some people with disabilities that involve fatigue or problems thinking. Some people don't find it helpful, but others do, and it's worth experimenting with - especially if your fatigue means that listening to audiobooks/podcasts is difficult. Slowing the audio down, for some people including me, brings it back into the realm of the possible.
If you're new to slowing audio down, it will sound really weird and drone-like for the first little while, but your brain will adapt. For me it only took about a half hour, and after that the audio sounds perfectly normal - it just takes me less energy to listen to so it's a lot more relaxing.
There are very few resources online which explain which apps let you slow audio down, so I've tried to put together this list. If you know of other apps, please let me know at firstname.lastname@example.org and I'll add your information to the list.
Links in this article take you to a page that has instructions about how to slow audio down using the specific app/platform I'm writing about.
Apple's Books app
You can slow audio down with Apple's Books app for iOS.
Pocket casts for iOS has awesome speed controls, enabling you to set 90%, 80%, 70%, 60% etc.
Apple podcasts for iOS does have a slow speed but it's 50% of normal speed which is too slow for me to get used to listening too. I've been asking them to add a 75% speed like they have in their Books app but it hasn't happened yet. If this is a thing you'd use, email email@example.com and let them know it's an accessibility issue - more emails will help.
Overcast for iOS used to offer a slow speed option but removed it sadly.
Commands such as "OK Google, repeat slowly" will get Google Assistant to repeat what it said, but more slowly.
For podcasts, news, and audiobooks "Play slower" (or "Play faster" if you didn't like the effect) seems to be the relevant command although I don't have a Google Assistant to test them.
You can change the default speed rate that Alexa speaks at using "Alexa speak slower", "Alexa speak faster", or "Alexa speak at your default rate".
Siri for iOS
With Siri, you can't change the general speed she uses to answer spoken queries, but you can change the speed used by Siri for speaking selected content, or speaking the screen.
If you use Safari for Mac, the extension Accelerate for Safari can also be used to set speeds slower than 100% to slow things down.
For Chrome users, may be of use.
National Library Service
The NLS provides spoken books and magazines to people who are blind or who can't hold and manipulate regular books - people who are disabled qualify. It's a free service and there are equivalents available in most countries (Vision Australia Library in Australia, and RNIB in the UK for example).
NLS provides physical players or an app called BARD Mobile, and I've been told that both of these methods allow the audio to be slowed down.