To start your free trial:
Sign on to your XBOX Live Gold account
Select MusicMarketplace/Zune from the dashboard
Select Zune pass tile and follow the on-screen instructions.

What's more, your Zune Pass isn't limited to your Xbox 360it's yours to use anywhere you have Zune. That means you can listen to streaming music using:

Zune on Xbox LIVE on Xbox 360
The Zune software on a PC
A Zune HD player
A Windows Phone 7, which means you can stream music anywhere you have a web browser with Internet access

You can also sign up from your computer. And it does not look like it is auto-renew, so you do not have to cancel.

Looks like a decent trial from Microsoft and worth a shot.

Link to sign up thru computer: Zune sign up