Ive tested this on Vanilla mIRC and confirmed the results.

If the user presses the RETURN key with a custom window active, mIRC will produce a windows beep. It does this with sounds off, it even does it with picture windows. This is incredibly annoying. My script catches the Enter key and uses it as a function for the script. However, every time a user presses enter, it generates a beep.

There does not appear to be a fix, and this bug did not appear in previous versions to 6.xx to my knowledge.

TO REPRODUCE:

Enter the following command:
---
/window p @test
---
-> This should produce a custom picture window WITHOUT an edit box, which may produce a beep with no text entered
---
Press Enter/Return
---
-> This should produce the annoying unstoppable beep.

There is no editbox, but it beeps anyway!

I've also sent this bug to khaled