Use SNDIO_DEBUG with the application you use to play the sound.
$ env SNDIO_DEBUG=1 play file.mp3
aucat_open: host= unit=0 devnum=0 opt=default
/tmp/aucat-1000/aucat0: No such file or directory
aucat: start, maxwrite = 0
file.mp3:
File Size: 4.52M Bit Rate: 128k
Encoding: MPEG audio
Channels: 2 @ 16-bit
Samplerate: 44100Hz
Replaygain: off
Duration: 00:04:42.96
In:0.00% 00:00:00.00 [00:04:42.96] Out:0 [ | ] Clip:0
aucat: flowctl = 8820, maxwrite = 35280
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
[...]
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
In:0.26% 00:00:00.74 [00:04:42.21] Out:32.8k [ -==|==== ] Hd:5.4 Clip:0
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
In:0.33% 00:00:00.93 [00:04:42.03] Out:41.0k [ -==|===== ] Hd:5.4 Clip:0
Aborted.
This shows that the sndio subsystem is indeed getting
audio samples from the application and tries to play them.
My bad. My output is:
$ SNDIO_DEBUG=1 ogg123 /tmp/songty.ogg
aucat_open: host= unit=0 devnum=0 opt=default
/tmp/aucat-1000/aucat0: No such file or directory
Audio Device: sndio audio output
Playing: /tmp/songty.ogg
Ogg Vorbis stream: 2 channel, 44100 Hz
aucat_open: host= unit=0 devnum=0 opt=default
/tmp/aucat-1000/aucat0: No such file or directory
aucat: start, maxwrite = 0
aucat: flowctl = 8820, maxwrite = 35280( 0.4 kbps) Output Buffer 44.4%
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528 ( 69.5 kbps) Output Buffer 77.8%
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
aucat: flowctl = 882, maxwrite = 3528
^Caucat: flowctl = 882, maxwrite = 3528
