YouTube doesn't provide a way to search all of YouTube based on the spoken words in videos, unfortunately.

I could update VideoMentions Search to allow users to select multiple channels, and then perform the search across all of those. Like maybe maybe auto-importing all the channels they're subscribed to could be useful. One way or another though, it would still require selecting specific channels to search within. For this first iteration, I just kept things simple with a single channel URL input. Despite that limitation, I still think it's a useful tool, though; I plan to use it often myself.

Thanks for checking it out!