Background music on an attachment

Vista Home Premium 32-bit SP1, Windows Live Mail 14 v.

I know how to insert background music on an e-mail, but ......................
I can not figure out how to make it as a background music when and ONLY when the attachment is opened.
NOTE : I do not want to convert the music to .wav format. It takes too long and it does not work if the music is too long.
What I have be doing is this :
Open compose window > Format > Background > Sound > browse to music folder, pick my choice > Open, choose # of times or continuously > OK
My problem is : as soon as the e-mail is opened by the recipient, music starts. It does not wait for attachment to be opened.
What steps have I missed ? Thank you.