Since there isn't a retail audiobook on platforms like Audible, the community has stepped in to fill the gap:

In conclusion, while there are some websites that offer free audiobook versions of Highschool DxD, be cautious when using these platforms. Make sure to check the quality of the audiobook and the website's legitimacy before using it. Alternatively, you can consider purchasing the audiobook from official platforms like Audible or Google Play Books.