Thanks for your bug report. I have not been able to reproduce this issue here yet. I don't use the insider preview builds - as they usually have too many issues - so I tested this on Windows 7, 10, and 11.
To test this, I ran mIRC, left it open on the desktop, and then 1) Locked Windows or 2) Put Windows into Sleep mode (it wasn't clear which of these was causing the issue for you). In both cases, when resuming Windows, mIRC was running as expected.
I can't think of a reason why this would happen other than a Windows/graphics driver issue.
If you install mIRC, using the portable option in the installer, into a clean, empty folder, and test that, do you see the same issue?
If you type the following into mIRC:
/timer 0 1 beep 1
so that it beeps once a second, and then lock the computer, at which point does it stop beeping?