I spent some time looking into this to see if I could make mIRC intercept rich editbox events in specific contexts to prevent the editbox from beeping. The method involved guessing at and reproducing the internal rich editbox contexts (some of which are a little murky) that trigger beeps and customizing the way mIRC handles editbox input to intercept them. Unfortunately, this approach has the potential to affect editbox behaviour and would likely cause more issues than it resolves. So unless I can find a simpler, clearer method, it does not look like this will be possible.
