I was actually hunting for these myself a couple months back for a road trip with my kid. The main Abby Hayes books, the 20-something original chapter books from the early 2000s, are a real mixed bag. You can find a handful as ebooks if you dig around major retailers, but the selection is spotty. The audiobook situation is even thinner; I think I found maybe one or two titles narrated, and the production felt pretty dated, like it was transferred from an old CD. It's frustrating because the series is perfect for that age group transitioning to longer reads.
Honestly, your best shot is checking used book sites for physical copies, which is a shame. I ended up reading some aloud from my old paperbacks and recording them myself on my phone for the car. Not ideal, but it worked. The publisher doesn't seem to have prioritized digitizing the whole set, which feels like a missed opportunity given how many of us 90s kids are now parents looking for them.