From mboxrd@z Thu Jan  1 00:00:00 1970
From: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Subject: Re: Join play of pcm in multiple playback devices
Date: Tue, 03 Jan 2012 15:32:23 -0600
Message-ID: <4F0373E7.2060807@linux.intel.com>
References: <4F029F10.7060704@sasken.com>
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> while(1)
> {
>       retval=snd_pcm_readi(pAlsaConfig->pcm_handle, buffer,
> pAlsaConfig->alsaBufferSize);
>       snd_pcm_writei(pAlsaConfig->pcm_handle1, buffer,retval);
>       snd_pcm_writei(pAlsaConfig->pcm_handle2, buffer,retval);
> }

This code doesn't work in the general case where the different devices 
don't share a common audio clock and have different physical 
capture/playback rates. Compensating for clock drift requires the use of 
asynchronous sample-rate conversion.
Assuming you have a perfectly synchronous system, you could make use of 
snd_pcm_delay() to compensate for delays incurred in the 
driver/firmware/hardware. This is not well supported by all drivers though.
-Pierre