Add sound customization

Add explanation for how to customize the default wakeup and timer sounds, and enable thinking sound on startup.

README.md

@@ -39,6 +39,27 @@ The microphone device **must** support 16Khz mono audio.
 
 Use `--audio-output-device` to change the speaker device. Use `--list-output-devices` to see the available speakers.
 
+### Sounds
+
+Customize wake word and timer sounds (defaults are used if not specified):
+``` sh
+python3 -m linux_voice_assistant ... \
+    --wakeup-sound sounds/wake_word_triggered_old.wav \
+    --timer-finished-sound sounds/timer_finished.flac
+```
+
+Available sounds:
+* **Wake sounds**: `wake_word_triggered.flac` (default), `wake_word_triggered_old.wav` (classic)
+* **Timer sounds**: `timer_finished.flac` (default), `timer_finished_old.wav` (classic)
+
+The optional "thinking" sound plays while the assistant is processing. Enable it on startup with:
+``` sh
+python3 -m linux_voice_assistant ... \
+    --enable-thinking-sound
+```
+
+This enables the thinking sound by default and sets the Home Assistant switch to ON. The switch can be toggled at any time from the device page.
+
 ## Wake Word
 
 Change the default wake word with `--wake-model <id>` where `<id>` is the name of a model in the `wakewords` directory. For example, `--wake-model hey_jarvis` will load `wakewords/hey_jarvis.tflite` by default.