Accessibility is one use case (probably the main one) but not the only one.
Since a forum is meant to facilitate meaningful interactions and build community, offering more than one way to share a thought seems valuable. I don’t know how complex this would be from a development perspective, or whether it would be worth the effort, but I keep thinking about how easily people express themselves in WhatsApp groups simply because the option to record a voice note is right there. It would be wonderful to have something similar in a forum, where conversations can be better organized by topics and categories.
I myself often use Instagram’s feature that transcribes voice notes instead of playing them. It’s convenient when I don’t want to use headphones, hold the phone to my ear, or play audio out loud. Having a similar option in a forum would make voice notes far more practical. And personal.
Of course, forums are not exactly designed for people who don’t like to read or write, or who find it too much trouble to switch between apps just to copy and paste transcribed text. But in my case, most people in the forum I manage access Discourse from their mobile devices rather than from computers. I imagine that this changes how and when they participate. Many might give up on posting simply because they can’t type much at that moment. Being able to speak and post a voice note, or even better, to transcribe speech to text and edit it later,would make a big difference.
On the other hand, voice notes make moderation more challenging, so this is something we need to think through collectively, considering as many different user scenarios as possible.