[fix](trx-server): pause audio playback stream when idle

Start the output stream paused and only play when TX packets
arrive. Pause again when the packet queue drains to prevent
continuous ALSA buffer underruns (EPIPE errno -32) on Linux.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-server/src/audio.rs

@@ -224,12 +224,19 @@ fn run_playback(
         None,
     )?;
 
-    stream.play()?;
-    info!("Audio playback: started");
+    // Start paused — only play when TX packets arrive
+    info!("Audio playback: ready ({}Hz, {} ch)", sample_rate, channels);
 
     let mut pcm_buf = vec![0f32; frame_samples];
+    let mut playing = false;
 
     while let Some(packet) = rx.blocking_recv() {
+        if !playing {
+            stream.play()?;
+            playing = true;
+            info!("Audio playback: started");
+        }
+
         match decoder.decode_float(&packet, &mut pcm_buf, false) {
             Ok(decoded) => {
                 let mut ring = ring_writer.lock().unwrap();
@@ -239,6 +246,18 @@ fn run_playback(
                 warn!("Opus decode error: {}", e);
             }
         }
+
+        // Pause when no more packets are queued to avoid ALSA underruns
+        if rx.is_empty() {
+            // Drain remaining samples before pausing
+            std::thread::sleep(std::time::Duration::from_millis(frame_duration_ms as u64 * 2));
+            if rx.is_empty() {
+                let _ = stream.pause();
+                playing = false;
+                ring_writer.lock().unwrap().clear();
+                info!("Audio playback: paused (idle)");
+            }
+        }
     }
 
     Ok(())