Q: Why is coreaudiod keeping my MBP awake?

Idle sleep is not working on my MBP. I went so far as to install PleaseSleep, and that won't work. I finally decided to dig into this, and looked at the assertions log with pmset:

pmset -g assertionslog

Logging all assertion changes.

9/27/11 9:15:51 AM EDT 

Assertion status system-wide:

   ChargeInhibit                           0

   PreventUserIdleDisplaySleep             0

   PreventUserIdleSystemSleep              1

   NoRealPowerSources_debug                0

   CPUBoundAssertion                       0

   EnableIdleSleep                         1

   PreventSystemSleep                      0

   DisableInflow                           0

   DisableLowPowerBatteryWarnings          0

   ExternalMedia                           0

Listed by owning process:

  pid 210: [0x0000012c000000d2] PreventUserIdleSystemSleep named: "com.apple.audio.'AppleHDAEngineOutput:8,0,1,1:0'.noidlesleep"

Kernel Assertions: None

pid 210 is coreaudiod. I can't for the life of me figure out why core audio would be preventing the Mac from sleeping. Any thoughts? I don't have any USB devices connected.

Thanks.