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.