Well Windows 7 comes preinstalled with Windows Media Player
12 but sadly, there are still radio streaming sites that require you to use Windows Media Player 11 which is an older version.
If you don't mind the advantages on Windows Media Player
12 over Windows Media Player 11 and if you really need this Sirius Internet Radio service so much because you are paying for it, you might as well consider downgrading to Windows Player 11.
Here is a link on how:
How to Downgrade Windows Media Player 12 to WMP 11 on Windows 7
But note that there are OTHER online radio streaming sites you can use that support Windows Media Player 12. There are tons of them.
OR you can simply wait till their radio streaming service to support Windows Media Player 12 since Windows 7 is getting more popular. You can mail them for comments and support too if possible.