Finding PCM audio devices using the ALSA interface from software isn’t always completely trivial. On some builds for the BeagleBone Black, PCM audio devices appear as /dev/audio, /dev/audio1 etc. But this doesn’t seem to be generally true. Plus, this doesn’t definitively identify which device is which. A good way to find out is to open the pseudo file /proc/asound/pcm as it lists all the PCM devices in the system. To see what’s currently active, just cat the pseudo file:

UbuntuDev$ cat /proc/asound/pcm
 00-00: ALC889 Analog : ALC889 Analog : playback 1 : capture 1
 00-01: ALC889 Digital : ALC889 Digital : playback 1
 00-02: ALC889 Analog : ALC889 Analog : capture 2
 00-03: HDMI 0 : HDMI 0 : playback 1
 01-00: USB Audio : USB Audio : capture 1
 02-03: HDMI 0 : HDMI 0 : playback 1
 02-07: HDMI 0 : HDMI 0 : playback 1
 02-08: HDMI 0 : HDMI 0 : playback 1
 02-09: HDMI 0 : HDMI 0 : playback 1

The first two numbers are the plughw card and device numbers. So, if you wanted to connect to the USB audio source for example, you could do:

snd_pcm_open (&m_handle, "plughw:1,0", SND_PCM_STREAM_CAPTURE, 0);