Clean mIRC, same issue.

Relatively recent install of XP SP2, no OS issues.

I'm trying to narrow it down a bit more. I just changed the sound for DCC Send, then clicked between each item on the list, in succession, and then back to DCC Send. Sometimes, clicking DCC Fail, then back to DCC Send will do it. Sometimes you need go almost all the way down the list (DCC Send > DCC Fail > DCC Send > Disconnect > DCC Send > Flash > DCC Send, and so on) for DCC Send to revert to Event Beep.