I was looking for it just last week, and it was weirdly tricky to track down. The audiobook for 'Phantom Infinite' isn't on some of the bigger platforms I checked first, like Audible or Google Play Books. I eventually found it exclusively on a subscription service called Scribd. It's part of their premium catalog, so if you have a membership, you can stream it directly. I hadn't used Scribd much before, but their app is actually pretty decent for listening.
What's interesting is that I don't think it was ever released as a standalone purchase on CD or for a one-time download. The narration is done by someone named Lia Carter, and she does a solid job with the protagonist's internal monologue, which is a huge part of the book's atmosphere. If you're not into subscription models, your only other legal option might be checking if your local library's digital service, like Libby or Hoopla, has it, but availability there seems really spotty.
I ended up using a free trial month on Scribd to listen to it. It's a shame it's not more widely available, because the story's dense, recursive structure actually works surprisingly well in audio format.