To overdub/mix record, open your audio file in WavePad. Then, start Overdub/Mix record by pressing Ctrl+Shift+F5 or by going to "Menu" -> "Control" -> "Record" -> "Record – Overdub/Mix". Recording will start immediately at your cursor, you will hear the audio file playing as you record your overdub. When you are finished recording, press stop.
If you need to start over, use undo to remove your previous recording, place your cursor at the start position, then start overdub /mix record again using the shortcut keys Ctrl+Shift+F5 or by going to "Menu" -> "Control" -> "Record" -> "Record – Overdub/Mix".