If you have high CPU usage while playing music (or using the sound card) this could be related to the sample rates of the sound card and your current alsa/pcm/dmix configuration. Probably your card is set to 48kHz and the music file is at 44.1kHz (or vice-versa) and this is making the CPU to re-sample from one to another unnecessarily therefore the high CPU usage.

I was able to solve this by setting my ~/.asoundrc config file to:

–

pcm.!default {
    type plug
    slave.pcm {
        type dmix
	ipc_key 1024
	slave {
		pcm "hw:0,0"
		rate 44100
	}
    }
}

–

This caused my CPU usage to drop from ~30-40% to 2-4%.

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Source: http://blog.sarine.nl/2008/12/21/dmix-the-pain-or-mpd-high-cpu-usage/