snd_uaudio: drop the default monitor level to 10%

As noted in the comment, some headsets with a hardware sidetone are
incredibly sensitive and emit immediate feedback upon attach with the
current system-wide default of 75%.  Drop it down just for snd_uaudio(4)
to avoid incredibly unpleasant surprises.

MFC after:	3 days
Reviewed by:	christos
Differential Revision:	https://reviews.freebsd.org/D59199

sys/dev/sound/usb/uaudio.c

@@ -112,6 +112,13 @@ SYSCTL_INT(_hw_usb_uaudio, OID_AUTO, default_channels, CTLFLAG_RWTUN,
 #define	UAUDIO_BUFFER_MS_MIN	1
 #define	UAUDIO_BUFFER_MS_MAX	8
 
+/*
+ * The default monitor level is high enough that headsets with a hardware
+ * sidetone may emit immediate feedback upon attach.  We'll lower the default
+ * just for snd_uaudio(4) to avoid breaking others.
+ */
+#define	UAUDIO_DEFAULT_MONITOR	10
+
 static int
 uaudio_buffer_ms_sysctl(SYSCTL_HANDLER_ARGS)
 {
@@ -1199,6 +1206,8 @@ uaudio_attach_sub(device_t dev, kobj_class_t mixer_class, kobj_class_t chan_clas
 	}
 	if (mixer_init(dev, mixer_class, sc))
 		goto detach;
+	mix_set(sc->sc_child[i].mixer_dev, SOUND_MIXER_MONITOR,
+	    UAUDIO_DEFAULT_MONITOR, UAUDIO_DEFAULT_MONITOR);
 	sc->sc_child[i].mixer_init = 1;
 
 	mixer_hwvol_init(dev);