TASK 9 audio milestone: real stream audio + voice filters + auto-duck + free TRAX music — the mixer now feeds the encoder real audio (ffmpeg reads a Windows named pipe, -f f32le -ar 48000 -ac 2 -i \.\pipe\ytllive_audio, replacing anullsrc silence; explicit -map 0:v -map 1:a via EncoderOptions.AudioPipeName), with honest gains reaching the stream (MicVolume scales mic, GameAudioVolume + mute scale loopback, Func<double> seams on AudioMixer), an always-on pure-C# voice chain on the mic BEFORE the meter and mix (LowShelfFilter 120Hz +4dB → HighShelfFilter 8kHz +3dB → NoiseGate 0.005/hysteresis 0.5 → Compressor 0.5 4:1, all TDF2), AutoDucker (mic RMS>0.02 → loopback ×0.25, attack 0.05/release 0.005), and TRAX free background music (MusicPlayer = MediaFoundationReader → VolumeWaveProvider16 at fixed 0.20 → WaveOutEvent via the new sibling NAudio.WinMM 2.2.1 package; plays to the default device so the existing loopback carries it, ducked with game; footer TRAX button with red/yellow/green status dot, left-click toggles/picks, right-click opens the OpenFileDialog picker, tooltip shows the track name; persisted via schema v9 single-row Music). Build-time deviations from the plan (recorded in ai.md + TASKS.md): WasapiCapture in NAudio 2.2.1 exposes no overridable GetDefaultMixFormat so sources run device mix format and the mixer's TinyResampler normalizes any rate to 48kHz (resampler IS the design); FfmpegEncoder.cs untouched — MainViewModel.StopStream calls _audioMixer.StopLive() (pipe EOF) before the frame pump stops; sound-bar relabelled 'Desktop Audio', IsGameAudioBarVisible = game sound OR music playing. Tests: new AudioPipelineTests (DSP/ring-buffer/ducker/resampler units + the ONE integration test Mix_WithFiltersDuckAndGain_Lands_On_AudioPipe reading real pipe bytes; ring-buffer overwrite bug found + fixed), FfmpegEncoderTests/LayoutStorePersistenceTests updated — 196 tests passing, 0 warnings

MainWindow.xaml

@@ -746,20 +746,20 @@
                             </StackPanel>
                         </Border>
 
-                        <!-- Game audio bar: overlaid at the bottom of the preview
-                             window while a full-screen game is producing sound
-                             (the game audio detector). Monitoring UI only — it
-                             lives in the XAML preview and never reaches the live
-                             output. Desktop audio itself is automatic WASAPI
-                             loopback. -->
+                        <!-- Desktop/game audio bar: overlaid at the bottom of the
+                             preview while a full-screen game is producing sound or
+                             TRAX music is playing (the game audio detector). Meter
+                             + mute + volume control live here; desktop audio itself
+                             is automatic WASAPI loopback, and since TASK 9 it rides
+                             into the live mix (game volume + mute are stream-honest). -->
                         <Border HorizontalAlignment="Center" VerticalAlignment="Bottom"
                                 Background="#D9000000" CornerRadius="6" Padding="12,8"
                                 Margin="0,0,0,12"
                                 Visibility="{Binding IsGameAudioBarVisible, Converter={StaticResource BoolToVis}}">
                             <StackPanel Orientation="Horizontal">
-                                <TextBlock Text="Game Audio Capture" Foreground="#a0a0b0" FontSize="11"
+                                <TextBlock Text="Desktop Audio" Foreground="#a0a0b0" FontSize="11"
                                            FontWeight="SemiBold" VerticalAlignment="Center" Margin="0,0,8,0"
-                                           ToolTip="Desktop/game audio via WASAPI loopback (automatic)"/>
+                                           ToolTip="Desktop/game audio via WASAPI loopback (automatic); TRAX music rides the same channel"/>
                                 <Border Width="288" Height="14" CornerRadius="7" Background="#3a3b52" Margin="0,0,8,0"
                                         ClipToBounds="True" VerticalAlignment="Center">
                                     <Grid>
@@ -896,6 +896,21 @@
                             <TextBlock Text="MIC"/>
                         </StackPanel>
                     </Button>
+                    <!-- TRAX (TASK 9): free background music. Dot: green = playing,
+                         yellow = loaded/stopped, red = no track. Left-click toggles
+                         play/pause (opens the picker when no track); right-click
+                         always opens the picker. -->
+                    <Button Style="{StaticResource YtButtonSecondary}" Padding="12,4"
+                            FontSize="11" VerticalAlignment="Center" Margin="0,0,8,0"
+                            ToolTip="{Binding TraxToolTip}"
+                            Click="TraxButton_Click"
+                            PreviewMouseRightButtonUp="TraxButton_PreviewMouseRightButtonUp">
+                        <StackPanel Orientation="Horizontal">
+                            <Ellipse Width="8" Height="8" Fill="{Binding TraxDotBrush}"
+                                     Margin="0,0,6,0" VerticalAlignment="Center"/>
+                            <TextBlock Text="TRAX"/>
+                        </StackPanel>
+                    </Button>
                     <!-- Sound meter: muted track with a ruler scale, zone tints at
                          the yellow (60%) and red (80%) starts. READ-ONLY realtime
                          level display (fill = live level scaled by volume). -->