So, how do we manage to reproduce the same beep as in previous versions?
You cannot, I have removed this option. mIRC now uses its internal beep sound, which has been the default sound in mIRC for about twenty years :-]
The reason I have removed this is that the non-internal beep sound that was being played was the Windows system default sound, which 1) is used by several other system events in Windows, such as asterisk, exclamation, system notification, and so on, which means it cannot be distinguished from these Windows events, and 2) is often a long-playing sound that can last several seconds and is not suitable for multiple quick beeps, ie. it gets cut off with each beep.
If you really want to set your own beep sound, I can add that to my to-do list for a future version.