Nick, sorry for the delay in answering - I'm overseas with work just now. Apologies to your wife for startling her!
The answer to your query is very similar to the answer to Yogi's question about the select sound:
Have a look in C:\ProgramData\Media Center Themer\Theme\. The elements that are changed can be found in the XML file, and the items themselves are mainly in \Resources\ehres\Images.
You have a few options:
The sound that you want to change is INTRO_A.
If you open up the XML, search for INTRO_A. Then just delete
<Replace File="Theme\Resources\Ehres\Images\INTRO_A.WAV" />
The sound then won’t be modified, and you'll get the default WMC intro sound.
you could just replace the INTRO_A wav in C:\ProgramData\Media Center Themer\Theme\Resources\ehres\Images with whatever wav file you want (you still need to call it INTRO_A.WAV, though).
you could replace the current INTRO_A with this (right click and "Save target as...) one, where I've cranked back the volume a bit for you. If you have a wav editor, you can adjust the volume to suit yourself.
Let me know how you get on.
Edited by keef, 02 November 2013 - 05:00 AM.