diag(audio): say why the Listening device is silent

"I turned the sound back on and nothing comes out of the speakers", with
a log that reports success at every step: the network audio stream up,
664-byte packets arriving, the monitor started. The render goroutine's
error was thrown away — a device unplugged, renamed by Windows or unable
to open at 16 kHz fails exactly there, silently.

It is logged now, with the device id. Two once-only lines either side of
it say whether the decoded audio is reaching a running monitor or
arriving with nobody listening, which separates a device fault from the
speakers simply being switched off.

internal/audio/manager.go

@@ -15,6 +15,12 @@ type Manager struct {
 	mu      sync.Mutex
 	recStop chan struct{}
 	recDone chan recResult
+	// Said once each: "the audio is reaching the speakers" and "it is arriving
+	// with nobody listening". Both are answers to the same evening — sound
+	// switched on, nothing out of the speakers — and neither is worth a line per
+	// packet at fifty packets a second.
+	gotAudioOnce sync.Once
+	noSinkOnce   sync.Once
 	// monGainPct scales what the RX monitor plays, 100 = as captured.
 	//
 	// The "From radio" slider used to reach only the QSO recorder, so an
@@ -224,6 +230,10 @@ func (m *Manager) StartMonitor(inputDev, outputDev string) error {
 	return m.startMonitor(inputDev, outputDev, true)
 }
 
+// Logf receives this package's diagnostic lines. Set to applog.Printf by the
+// app; a no-op in tests and for anything that vendors the package alone.
+var Logf = func(string, ...any) {}
+
 // StartMonitorSink starts ONLY the render side (no USB capture) so an external
 // producer — the network 50003 stream — can feed decoded RX PCM via
 // PushMonitorAudio. Same output path as StartMonitor, minus the capture goroutine.
@@ -255,8 +265,17 @@ func (m *Manager) startMonitor(inputDev, outputDev string, capture bool) error {
 		}()
 	}
 	// Consumer: render the ring to the output device at the internal 16 kHz mono.
+	//
+	// The error was thrown away, and that is the whole of "I turned the sound on
+	// and nothing comes out": a Listening device that has been unplugged, renamed
+	// by Windows or cannot open at 16 kHz fails here, silently, while everything
+	// upstream reports success — the stream is up, the packets arrive, the
+	// monitor says it started. Said out loud, the operator knows to look at the
+	// device rather than at the radio.
 	go func() {
-		_ = renderStream(outputDev, sampleRate, channels, bitsPerSample, stop, ring)
+		if err := renderStream(outputDev, sampleRate, channels, bitsPerSample, stop, ring); err != nil {
+			Logf("audio: the Listening device could not be opened (%q): %v", outputDev, err)
+		}
 	}()
 	m.notify()
 	return nil
@@ -321,9 +340,19 @@ func (m *Manager) PushMonitorAudio(pcm []byte) {
 	m.mu.Lock()
 	ring := m.monRing
 	m.mu.Unlock()
-	if ring != nil {
-		ring.Push(pcm)
+	if ring == nil {
+		// Nothing is listening: the stream is feeding the recorder and the voice
+		// keyer only. Said ONCE, because the alternative — silence in the log for
+		// silence in the speakers — is what makes this take an evening to find.
+		m.noSinkOnce.Do(func() {
+			Logf("audio: network RX audio is arriving but no monitor is running — the speakers are off (Listening)")
+		})
+		return
 	}
+	m.gotAudioOnce.Do(func() {
+		Logf("audio: network RX audio reaching the Listening device (%d-byte chunks)", len(pcm))
+	})
+	ring.Push(pcm)
 }
 
 // ---- TX audio passthrough (Phase 3: live mic → rig over USB) --------------