mIRC 6.2 (Haven't tested prior versions)
1. Went through and changed all the sound events in the sound options (dcc send, dcc fail, etc).
2. Clicked Ok
3. Went back into sound events and toggled through each event.
If I toggle the dropdown between DCC Send and other events enough (DCC Fail, Flash, Disconnect, Highlight, etc), it will reset DCC Send to "Event Beep" as long as the options are open.
You can also see this when initially setting the sound events as well. As you click through the options, all of your other changes stay, but DCC Send will continually reset to Event Beep if you switch between them enough.