diff --git a/src/net/torvald/terrarum/audio/AudioMixer.kt b/src/net/torvald/terrarum/audio/AudioMixer.kt
index 0f92ea461..55827bfe5 100644
--- a/src/net/torvald/terrarum/audio/AudioMixer.kt
+++ b/src/net/torvald/terrarum/audio/AudioMixer.kt
@@ -123,14 +123,19 @@ class AudioMixer : Disposable {
      * Return oldest dynamic track, even if the track is currently playing
      */
     fun getFreeTrackNoMatterWhat(): TerrarumAudioMixerTrack {
-        return getFreeTrack() ?: dynamicTracks.minBy { it.playStartedTime }
+        synchronized(this) {
+            return getFreeTrack() ?: dynamicTracks.minBy { it.playStartedTime }
+        }
     }
 
     /**
      * Return oldest dynamic track that is not playing
      */
     fun getFreeTrack(): TerrarumAudioMixerTrack? {
-        return dynamicTracks.filter { it.trackingTarget == null && !it.isPlaying }.minByOrNull { it.playStartedTime }
+        synchronized(this) {
+            return dynamicTracks.filter { it.trackingTarget == null && !it.isPlaying }
+                .minByOrNull { it.playStartedTime }
+        }
     }
 
     var listenerHeadSize = BinoPan.EARDIST_DEFAULT; private set
diff --git a/src/net/torvald/terrarum/audio/TerrarumAudioMixerTrack.kt b/src/net/torvald/terrarum/audio/TerrarumAudioMixerTrack.kt
index d71a02782..87fdf86d7 100644
--- a/src/net/torvald/terrarum/audio/TerrarumAudioMixerTrack.kt
+++ b/src/net/torvald/terrarum/audio/TerrarumAudioMixerTrack.kt
@@ -162,7 +162,8 @@ class TerrarumAudioMixerTrack(
     override fun equals(other: Any?) = this.hash == (other as TerrarumAudioMixerTrack).hash
 
     fun stop() {
-        printdbg(this, "Stop music (mixertrack=${this.name}, musictrack=$currentTrack)")
+        printdbg("TerrarumAudioMixerTrack $name", "Stop music (mixertrack=${this.name}, musictrack=$currentTrack)")
+        printStackTrace("TerrarumAudioMixerTrack $name")
 
         currentTrack?.reset()
 
diff --git a/src/net/torvald/terrarum/modulebasegame/gameactors/FixtureSmelterBasic.kt b/src/net/torvald/terrarum/modulebasegame/gameactors/FixtureSmelterBasic.kt
index a60cb957b..697074d37 100644
--- a/src/net/torvald/terrarum/modulebasegame/gameactors/FixtureSmelterBasic.kt
+++ b/src/net/torvald/terrarum/modulebasegame/gameactors/FixtureSmelterBasic.kt
@@ -289,6 +289,7 @@ class FixtureSmelterBasic : FixtureBase, CraftingStation {
 
 
         // manage audio
+        // FIXME this code is also killing the audio played by the other fixture (FixtureFurnaceAndAnvil)
         getTrackByAudio(static).let {
             if (it == null || (temperature > 0f && !it.isPlaying && !it.playRequested.get())) {
                 startAudio(static) {