pulseaudio Invalid argument error

When using qemu-system-ppc on Ubuntu Mate 15 with the usb audio card, I see these error messages:

pulseaudio: set_sink_input_volume() failed
pulseaudio: Reason: Invalid argument
pulseaudio: set_sink_input_mute() failed
pulseaudio: Reason: Invalid argument

No audio plays. When an attempt is made, QEMU seems to freeze for a moment. 

I use "-device usb-audio" to add the usb sound card. This issue is present in both emulation and KVM mode.