Hi there!! Long answer ahead but I feel like I could provide some insight. I (22F) caught my husband (26M) messaging with people online who would share their selfwritten erotica. I was very upset because for one it was a secret and two it felt almost like he was cheating. Looking for something outside of our marriage that I couldn't fulfill sexually, even if it was just a story written by a stranger. It can be very hard to come to terms with something like this, he's an avid porn watcher but for some reason it just felt... Different. A feeling I'm sure your partner could be experiencing as well.
That all being said, I think if it's something that really matters to you then just talk to your partner. Being on those websites or wherever you get your audio from, it can be very tempting to comment or start a conversation with the creator to compliment or discuss their work. Speaking from experience, it hurts so much more after finding out by accident that your partner is keeping secrets and crossing boundaries within your relationship.
As someone who's been through a similar situation and had to talk to their partner about it, I can say it was a very hard conversation to have but in the end I understood him and his desires all the better for it. When you're in a relationship it can be very easy to get caught up in trying to be anything and everything your partner needs, and the reality is that no one person can provide you all of those things. The key to making it work I think is to sit down and discuss. Be vulnerable and share you feelings with your partner, while also being kind and considerate of their feelings as well. It might be a hard conversation to have, but if they're the right person for you, you'll be able to work together to find a compromise. Just remember to listen with the intent to understand and speak from love. It sounds like you deeply care for your partner and don't want them to be hurt. I think if you share that sentiment and also express your needs and the reasons behind them it could benefit you.