Audio volume setting: not working - sometimes?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

9.1, amd64.  Audio hardware is

[     1.047133] hdaudio0 at pci0 dev 31 function 3: HD Audio Controller
[     1.047133] hdaudio0: interrupting at msi5 vec 0
[     1.047133] hdafg0 at hdaudio0: vendor 10ec product 0269
[     1.047133] hdafg0: DAC00 2ch: HP Out [Jack]
[     1.047133] hdafg0: DAC01 2ch: Speaker [Built-In]
[     1.047133] hdafg0: ADC02 2ch: Mic In [Jack]
[     1.047133] hdafg0: 2ch/2ch 32000Hz 44100Hz 48000Hz 88200Hz 96000Hz 192000Hz PCM16 PCM20 PCM24 AC3
[     1.047133] audio0 at hdafg0: playback, capture, full duplex, independent
[     1.047133] audio0: slinear_le:16 2ch 48000Hz, blk 1920 bytes (10ms) for playback
[     1.047133] audio0: slinear_le:16 2ch 48000Hz, blk 1920 bytes (10ms) for recording
[     1.047133] spkr0 at audio0: PC Speaker (synthesized)
[     1.047133] wsbell at spkr0 not configured
[     1.047133] hdafg1 at hdaudio0: vendor 8086 product 280b
[     1.047133] hdafg1: DP00 8ch: Digital Out [Jack]
[     1.047133] hdafg1: 8ch/0ch 48000Hz PCM16*

Userland does

 fd = open("/dev/audio0",O_RDWR,0);
...
 setup_audio();

static void setup_audio(void)
{
 struct audio_info oai;
 struct audio_info ai;

 fprintf(stderr,"%s\n",__func__);
 if (ioctl(fd,AUDIO_GETBUFINFO,&oai) < 0)
  { fprintf(stderr,"AUDIO_GETBUFINFO: %s\n",strerror(errno));
    exit(1);
  }
 AUDIO_INITINFO(&ai);
 ai.play.sample_rate = 8000;
 ai.play.channels = 1;
 ai.play.precision = 16;
 ai.play.encoding = AUDIO_ENCODING_ULINEAR_LE;
 ai.play.gain = AUDIO_MAX_GAIN;
 ai.play.port = oai.play.avail_ports;
 ai.play.pause = 0;
 ai.play.balance = AUDIO_MID_BALANCE;
 ai.monitor_gain = AUDIO_MAX_GAIN;
 ai.mode = AUMODE_PLAY | AUMODE_PLAY_ALL;
 if (ioctl(fd,AUDIO_SETINFO,&ai) < 0)
  { fprintf(stderr,"AUDIO_SETINFO: %s\n",strerror(errno));
    exit(1);
  }
}

and then, 8000 times a second, does

 unsigned char sample[2];

...
 sample[0] = v;
 sample[1] = v;
 write(fd,&sample[0],2);

(v coming in to that code is 8-bit unsigned linear).

This works fine, in that I get sound.  But it looks as though the
setting of ai.play.gain is being ignored: audioctl play.gain reports
the last value set with audioctl -w play.gain=..., even if I restart
the code I quoted above (and the printfs appear); the volume of the
sound driven out the jack matches what audioctl reports.  But I know
the AUDIO_SETINFO isn't simply being ignored, because the sample rate,
channel count, and precision change to 8000/1/16 even if they were (as
reported by audioctl -a) set differently before.

So, my question is, why is play.gain being ignored?  What am I doing
wrong?  I'm sure it's something that'll be obvious once I see it, but
I'm having trouble seeing it.

/~\ The ASCII				  Mouse
\ / Ribbon Campaign
 X  Against HTML		mouse%rodents-montreal.org@localhost
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B