Well, I understand your point, but it’s not quite the same nature of medium.
In a forum we are generally sharing images created by someone else, but voice notes usually contain your own voice in a more personal way. At least this is the objective.
The effort involved in producing your own photos, videos, and voice notes is quite different — and voice notes are usually the quickest and most direct way to share a thought. Taking or selecting a photo, or recording and uploading a video, often requires more steps.
But with audio, especially if the feature is embedded in the tool you are already using, you can simply speak and send. It’s a lower barrier to contribution, which is why I see value in having that option in a forum setting.
I even see more value in the capability of transcribing and having both, audio and text, because of the moderation and search challenges the audio only could bring.