[Bug 31011] Don't show "Default sound card" unless there is more than one card

[Crispin Flowerday]

Public bug reported:
https://launchpad.net/malone/bugs/31011

Affects: control-center (Ubuntu)
       Severity: Normal
       Priority: (none set)
         Status: Unconfirmed

Description:
There is no point in the "Default sound card" option unless the computer
actually has more than one sound card, this will have the advantage of
shrinking the dialogue for most users.