Better late than never! ;-)
Excerpt from www.advertsongs.co.uk site:

It's from the Outback Steakhouse ads on the radio, it's in the style of Australian digeridoo music and they always play it around Mother's Day. It goes something like: No one ever loves you like your mum (mummmm) We know a mother's work is never done (mumm) even when she whacks you on the bum (mummm) You will always be our number one (mummm) Mum's the word and so the word is Mummmm.

*Elektra*